I would like to put together an intercom system for my house. I want to be able to connect multiple stations, perhaps up to 10 or 12. I'd like to use twisted pair wiring, not shielded wire (for example, existing two pair telephone or four pair CAT 5). I would like to use a central power supply rather than individual batteries or power supplies. I don't need hands-free operation, so a simple push to talk function would be fine, and I'm not concerned about privacy, so when one station is talking, all stations would hear the conversation.
I've seen some two station circuits using LM386 but I can find nothing about creating a multi-station system.
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Considering the installation of a 'multi-station' intercom system wired throughout your premises can be somewhat of a task and perhaps a little more expensive than necessary as compared to the variety of 'wireless' intercom and phone systems already in use.
I opted to use a Uniden DECT 6.0 three-handset cordless phone system that offers excellent long-range wireless phone and 'conferencing' ability, selective between any/all three cordless handsets anywhere around my premises.
Although there are Uniden cordless phone units available with more handsets, this system can be purchased for a very reasonable price almost anywhere electronic equipment is sold. I carry one handset with me to any remote area, such as the garage, working in the yard, etc., and when my handset starts 'chirping' I can immediately identify [who] which handset is signaling me for conversation and, even if you don't connect to the phone line through the 'base set', this system can still be used as a long-range intercom system.
The intercom system desired is very similar to the headset intercoms used to communicate with camera operators and other studio personnel in every TV station and network for many years now. Although there are variations and expansions, the basis of these systems is a two-wire party line that carries both the conversation and the power (24 to 48 volts DC) to run the various stations so that no separate power connection is needed. They are probably similar to the two-station intercom circuits you have seen and many of these can be adopted for this use with the additional components shown in Figure 1. The basic idea is that each station must separate the audio and power components that are sent over the same two wires. Also, the audio must be blocked from entering the output terminals of the DC power supply because the filter capacitors in it will have a very low AC impedance and would short the audio to ground. There were also party line systems that used a four-wire connection to keep these two components separate, but I will ignore them here because a two-wire system was requested.
One wire is a signal and power ground, while the other carries both the audio and power. The basic audio circuit is simply an amplifier connected to the two-wire party line through a simple two-pole momentary switch which allows only one of these functions to occur at a time, thus preventing any feedback. I have shown a DPDT in the schematic to allow the speaker to be used as a microphone, but a separate mic could be used with a SPDT switch.
The coil labeled L1 is necessary to present a high impedance to audio frequencies on the party line which would be shorted by the filter capacitors in the power supply. It needs to have a fairly high inductance value in order to preserve the lower audio frequencies. You need whole Henries here, not milli or micro Henries. The formula for the impedance is 2*Pi*F*L and you want an inductive impedance value in thousands of ohms at about 100 Hz or lower. At 100 Hz for a 10 Henry (H) inductor: 2*3.141*100Hz*10H = 6283 ohms. This should work. It should be rated for the combined current of all the stations. The largest inductor I could find with a quick search is a 15 H coil but it was shown as "out of stock" with a delivery time of over 100 days. There are some being offered on eBay; search for "retard coil" and you will get what you need, but they are not cheap. There are high inductance chip-style inductors but I doubt they would work because they would not pass the DC current needed to operate all the stations.
In the stations, the capacitor labeled C1 blocks the DC voltage to the audio amplifier. It must have a voltage rating that is higher than the power supply output voltage and a low capacitive reactance in relation to the amplifier's input impedance. The formula for the capacitive reactance is 1/(2*3.141*F*C). If the amplifier you use has an input impedance of 10K ohms and you have 10 intercom stations, the combined parallel impedance on the party line will be 1,000 ohms; you will want a capacitive reactance of 100 ohms or less. Again, we calculate at the lowest desired frequency or about 100 Hz. A 15 µF capacitor will give us 106 ohms of capacitive reactance at 100 Hz and should work. I would probably step up to 25 µF or even 50 µf as the added cost will be tolerable and the low frequency performance will be improved.
Any of a number of solid-state or IC audio amplifier circuits could be used. Look for a high input impedance and a low output impedance suitable for driving your speaker. The gain needed will depend on the output level of the microphone or of the speaker when used as a mic and the desired line level on the party line. Since you want to use unshielded wire, I would suggest a line level of about +10 dBm or about three volts RMS in order to keep any noise to a minimum. Assuming the microphone level is -40 dBm, you would need a +50 dBm gain in the amplifier. I would go for one that provides +60 dBm to allow the volume controls room to work. The volume controls can be simple voltage division circuits with one side of the pot connected to the input signal, the other side to ground, and the output taken from the wiper; 10K or 50K audio taper pots should work with most types of amplifiers.
The figure below shows the DC power supply to have an output voltage that is significantly higher than the regulated voltages in the stations. This is because there are no filter capacitors on the party line. So, the regulator circuits in the stations must have sufficient voltage to stay in regulation, even during the loudest audio which will both add to and subtract from the average DC voltage provided by the power supply. So, with a three volt RMS audio level, half of the P-P value of the audio will be about 4.5 volts. This — on top of a supply voltage of 25 volts DC — will give a total voltage which will swing from 20.5 to 29.5 volts. If the regulator needs a three volt headroom to operate, this is subtracted from the minimum value of 20.5 to give a maximum regulated voltage of 17.5 volts. An 18 volt regulator would fall out of regulation on loud audio and cause distortion in the amplifier. Thus, there is a need for an unregulated voltage that is about 10 volts higher than the output of the regulators chosen.
Finally, a word about the type of wiring you wish to use. Telephone wires and Cat 5 network wires are relatively small gauge, so the power current will be somewhat limited. If you have multiple stations on a single run of such wire, the current needed for a single station will be multiplied by the number of stations on the run. This may cause problems with excessive voltage drops. Also, longer runs will add to this problem due to increased resistance in the wiring. The systems in TV stations I referred to earlier commonly only have to power headsets — not speakers — so less power is needed. I would suggest heavier gauge wiring; common lamp cord or even doorbell wire may work better. If you must use telephone or Cat 5 wiring, they commonly have four conductors and you should consider doubling up for both the ground and the signal/power conductors.
Think wireless!!! Go to gadgetshack.com and look at the Westinghouse two-channel basic home intercom system. For $59 per pair of units and a reputed "unlimited number" of units that can be used with full security, this is a lot cheaper and less of a "pain" than running cable. Running cable in an existing house is very difficult and installing cable in a new house is still expensive.
I have two different bathroom vents going into the same exhaust fan. I want to sense either or both switches to access the main relay to run the fan (kind of like an OR gate which I can make from logic gates for DC).
However, my situation calls for AC switches and fans, and I cannot think of a way without converting to DC to be able to switch the AC (long way around).
Is there a simple solution using two 110 VAC switches to turn on the exhaust fan, and then off when both switches are off?
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Of course, you can use two three-way AC switches or double pole switches. It is easily available in market. If you have the knowledge of electric powers than you can also do easily yourself, otherwise call electrician. You can get here https://aashirwadinterior.com/services/industrial-electrical-contractors-vadodara-ahmedabad/
The problem with standard SPST switches connected in parallel, if either is left on, the other cannot turn off the fan. If the switches are connected in series, when one switch is left off, the other cannot turn on the fan. The solution is a three way switch arrangement.
This uses two SPDT switches instead of the SPST version. Either switch can turn on or off the exaust fan. If you are not familiar with 3-way switch wiring, go to your home repair store and get a book like "Home Wiring For Beginners".
There is one problem, if someone turns on the fan in the first room, someone in the second room can turn it off if they did not notice the fan was running.
The answer to question # 2124 "Two AC Switches, One Fan" by Stan Strom on page 80 of the March 2012 edition of Nuts and Volts will not meet the questioners needs.
The circuit shown in the figure is a neat implementation of an inverted eXclusive OR (XOR) circuit using switches. In this circuit the fan relay will activate (thus turning the fan ON) when BOTH switches are in the ON position or BOTH switches are in the OFF position (XOR output is high if either input is high BUT not both inputs high). With the switches in opposite positions (ON-OFF states) the relay will deactivate and turn the fan OFF. [I have had students try to use switches to simulate logic gates like this. Good job Stan!]
Looking at 120 VAC bathroom fan specs, most draw 0.8 to 1.5 amps. If you have a light fixture with two (2) 60-watt bulbs, they will draw 1 amp. The 14-gage (14 AWG) wire used in most lighting circuits will handle 15 amps at 120 VAC and most lighting circuits are on 15 amp breakers. [Be sure to check that the bathroom lighting circuit is capable of handling the extra fan load just in case the installer was cutting corners]
Most vent fans are powered from a 120 VAC switch without a relay. I would locate the light fixture junction box and simply hook one 14 AWG wire from the HOT side of the lamp (its the BLACK wire if the light was wired in standard fashion, if the fan does not run when the switch id ON use the other wire at the light fixture) in each bathroom to the hot terminal of the fan. The vent fan neutral is hooked to the lighting circuit neutral. This hookup avoids using two (2) extra switches and a relay. One of the tenets of reliability says: "The more parts in a circuit, the more probable the system will fail." A tenet of economics says: "The more parts in a circuit, the more the circuit will cost."
If you want separate switches for the fans and lights, just wire the fan switches like the light switches but run the HOT wires from the fan switches to the HOT terminal of the vent fan. Two switches in parallel turned ON at the same time will not hurt anything, they will each carry half of the fan motor's current.
The original question mentioned using logic gates. The logic circuit would be simple: Hook the HOT wires from the light switch to the OR gate inputs through a device that converts 120 VAC to logic level voltage (e.g, an opto-isolator) and hook the OR gate output to the vent fan HOT terminal through a relay (logic voltage rated coil and 120 VAC rated Normally Open contacts). This circuit would have a reduced reliability and higher cost due to the high parts count. Engineers try to minimize parts count whenever possible.
On the mechanical side, be sure to vent the bathroom fan's exhaust to the outside to avoid mold and mildew in the attic. Also check all electrical and building codes to make sure you do not anger the "regulatory gods."
Sorry for the wordiness but engineers try to cover all of the possibilities.
All that is necessary to make two switches control a single relay for your bathroom fans are two single pole double throw switches. They're very often sold as three-way switches for controlling a lamp with two switches and are readily available at Lowes or Home Depot.
Feed the hot line (the black mains wire) to the common terminal of one of the switches; the two legs of the switches connect to one another — these are called 'traveler' lines. The relay connects between the common of the second switch and the neutral line (white mains wire).
Using the circuit below, if the relay is energized, then flipping either switch will turn it off. Then with the relay off, flipping either switch will turn it on. It is shown in the on position.
Maybe I'm missing your intent, but it seems you should be able to just wire two 120 VAC switches in parallel. Either OR both switches on will turn on the fan. Both switches off will turn off the fan.
I can see no reason not to connect two switches to one fan, that is, in parallel, as long as you check that the same circuit provides the "hot" lead to each switch. The US standard is Up = On, so it would be relatively easy to see which switch was used to turn on the fan.
If you are rewiring anyway, I would use single pole double throw switches as they are used when controlling a light with two or more switches. Then flipping any one of the switches will change the mode off to on or on to off. This requires that some of the cables have a third conductor known as a traveler.
If I understand this, then all you need are 2 three-way wall switches. One at each location of the two vents and then either switch will control the fan.
A simple solution is to use a three-way switch at each of the two locations (connected with a 3 wire cable) to control the Fan. Most wall switches are “single pole, single throw”. Outwardly, three-way wall switches” look the same, however they are single pole, double throw. Analogous to your situation, a common three-way switch use is to have one at the top & one at the bottom of a stair connected to control a light on the stairway.
Three-way wall switches” are inexpensive, easily found at home centers, electrical and hardware stores. The wiring is not complicated and there are many Internet sites for details, e.g., www.electrical-online.com/electrical/diagrams/
We have three bathrooms and one exhaust fan which is ducted to all three rooms. Our solution was to use a double pole switch in each room (commonly available in regular toggle wall switch format) where one pole does the lights in that room, and the other pole is wired in parallel with the second pole of the other two switches, and then to the fan motor, so that if any of the three switches is turned on, the fan runs, as well as the light in only that room turns on. Be sure to take safety all precautions, and follow electrical codes, as we are dealing with 110 VAC.
Use two three-way AC switches. These are very commonly used for lights so two switches can control the light. It will allow switching the fan off from either location no matter which location turned it on.
The problem might be getting the wire from switch #1 to switch #2 but since they both control the same fan there should be a way to get the needed wire from one to the other.
The switch instructions that should come along with them, or a good "How To" book will show how to wire them up correctly.
I’m just getting started in electronics and need advice on what kinds of tools I should get to make my projects easier.
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Start with a soldering kit (such as RadioShack's Cat. No. 64-2803), plus a couple of rolls of solder removal braid and a roll of rosin core solder (the RoHS stuff is "greener" but it's a pain even for seasoned veterans). If you plan to remove a lot of components from scrapped boards, a vacuum desoldering tool will keep your hair on your head.
I learned to solder at eight years old (52 years ago) with my grandfather's 250 watt soldering gun and a roll of solder that looked like it could be used to solder pipes, so the low wattage soldering irons used today are a snap. At 10 years old, my grandfather gave me a guitar amp. I learned not to troubleshoot the black tubes by touching them to see if they were warm (fried a finger tip in the process).
CAUTION: Wear safety goggles or glasses when soldering and don't even think about surface-mount devices yet.
Practice soldering by attaching two small wires together and then move up to a kit. Ramsey Electronics (www.ramseykits.com) has a lot of neat electronics kits for both beginners and old pros. Later, you will build some of your own designs or those from the authors of Nuts & Volts, and will need components; Jameco (www.jameco.com), Mouser (www.mouser.com), and Digi-Key (www.digikey.com) are good sources. Don't worry about microcontrollers now, but later you may try them and catch the programming bug.
Good luck, keep trying new things, and don't give up. Before you know it, you'll be an old pro too.
I found hundreds of caps inside a broken computer I’ve taken apart, but have no idea what they are all used for. I know that they store energy, however, I want to hear what you can do with them.
Is it just timing and smoothing? I’m looking to create circuits, and am puzzled by them.
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Old circuit boards are very useful for parts to build new projects. The motherboard caps are small with good capacity however you should check each capacitor for the correct value before using it! A common failure mode for motherboards is/are the capacitors swelling and going bad or venting.
One can use capacitors for filtering a power supply, bypassing an IC, in timing circuits or isolating a higher voltage. You can also use a large value capacitor to clear the shorting whiskers in a Ni-Cad battery. MUCH better than shorting out your power supply!
What connectors seem to be the choice of electronics hobbyists when assembling electronics projects on perf board? Also, is there a standard regarding the spacing of the "perfs" on the boards?
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I am a musician who uses several stomp boxes to change the sounds of my string musical instruments. Some of these effect units have as many as 80 different modifications available, of which I use only a few. I need an electronic device which will give an output of several different sound wave forms i.e., sine wave, triangle wave, sawtooth, etc., which will give me a constant level to use as I set up the various effects. There should be a meter or other read-out to adjust the level of the input and output so each effect is at the same amplitude. There also needs to be a switch which can select a burst of sound in addition to a constant signal to test the attack and decay of a sound.
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The easiest solution is to use a PC or laptop with an appropriate program such as: www.tucows.com/preview/240287 or www.xmarks.com/site/www.world-voices.com/software/nchtone.html. I found these by searching for "PC tone generator".
I'm looking for a cable that sends an input to the PLC when touched. Any suggestions?
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A programmable thermostat switches a 24 VDC relay to control 240 VAC electric heaters. I would like to know when and for how long the heaters are on and off in a 24 hour time span, although 12 hours would be okay. Is there such a thing as the electronic equivalent of a chart recorder that can display via Windows XP, and be built for less than a small fortune in time and money?
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Assuming you're not interested in a permanent monitoring solution, but wish only to take occasional measurements, a fast and inexpensive solution is to use a multimeter that has an RS232 interface and a temperature probe. One such instrument is the UT-61C available for $60 from www. multimeterwarehouse.com. Attach the temperature probe to the cover of the baseboard heater (not the heater element itself), and record the temperature variations over time on your PC using the software included with the multimeter. When you're done, you have a multimeter (although not the highest quality) to use for other projects.
If you are interested in knowing how long the heaters were running for the least amount of time and money, I would suggest using a panel mounted hour meter. You can get the meters from Digi-Key, Allied Electronics, or any well stocked supplier. I would suggest getting a meter that responds to the 24 VDC relay coil voltage. The one advantage is that you can get a cumulative reading of the time the relay is energized without having to utilize a computer. While a computer program is nice, if you should take a power bump your computer may shut off and you lose the data you were trying to get. Since the meter is directly powered, if you lose power and then it is restored the meter will keep working.
If you are up to messing with a microcontroller, I have a solution based on using an Arduino and a Flash memory card to save the data. To better flesh out the project, I have created a web page with a description (I did bench test it) and schematics: https://sites.google.com/site/thermostattracker/home.
Basically, you use a small front end to transform the thermostat signal for the Arduino microcontroller input, run a program on the Arduino to collect the data, and store it on a Flash memory card that is plugged into a daughter card (referred to as a "shield") on the Arduino. Then, you can take the Flash memory card and plug it into a PC for post-collection data analysis (example, import to a spreadsheet). I'd estimate about $75 for the parts; almost all are available at a RadioShack. I also used the free Arduino simulator available at arduino.com.au to aid in the programming.
I need a circuit that will read a person's heart rate by gripping two electrodes.
I have an "older" treadmill and find the clip-on ear heart rate monitor too unreliable. The newer treadmills use hand grips to measure pulse rate.
The circuit should use readily available components such as a digital voltmeter as a readout device. Also, the cost should be low (there are commercial devices costing over a $100).
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I am an electrical technician looking for a textbook or reference that deals with landline phones. I am wanting to learn the info that a phone technician would use such as multi-phone land lines, troubleshooting phone lines, and the definitions that are used in the industry.
Please direct me to a text book that would be a useful resource of learning and a good reference for down the road.
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Try:www.kadiak.org/tel/
Or just search the web using, "Telephone Technical Reference" to find a bunch more.
For a book on telephone electronics, see:
ISBN-10: 0750699442
ISBN-13: 978-0750699440
You can find them for about $10 new. There is even a Kindle edition, although it is close to $35.
