Interfacing with 120 VAC

In the main, I have confined discussions of printer port control to the control of devices requiring +12 VDC or less and have avoided discussing the control of devices such as lamps, AC motors and appliances. I have stayed away from discussions of 120 VA C as it is lethal. It is very dangerous.

Caution. 120 VAC kills. At Morgan State University, no student is to ever work with 120 VAC, except under the direct supervision of a faculty member. My suggestion to anyone is to always work with someone who is familiar with 120 VAC. Never wor k with live AC. Check and recheck to be sure the circuit is dry.

Prior to applying power for testing, be sure all is clear of any exposed power points and you are well aware of where the dangerous points are. Accidents are not planned, and it is not uncommon to "believe" you are getting a shock, you make a sudden move ment and do indeed come in contact with AC.

Use of an In-Line Cadmium Sulfide Controlled AC Light Controller.

Refer to the figure titled "Use of an In-Line Cadmium Sulfide Controlled AC Light Controller".

This is the safest approach as no AC wiring is required.

Radio Shack (Part 61-2778, $7.99) and I assume a number of other vendors offer an in-line product which requires no modification of 120 VAC wiring. The idea is that a homeowner who desires to upgrade their outside lighting to automatically turn on at dus k, simply screws the in line device in the lamp socket and then screws the bulb into the other side of the device. The device itself consists of a cadmium sulfide resistor and AC switch. When the ambient light is high, the switch is off and when dark, t he AC switch turns on and lights the lamp.

Many of my students have used this to control AC by simply glueing an LED to the "electric eye" as shown in the figure. The LED is then controlled by the printer port. When the LED is off, the device turns on the AC, when the LED is turned off, the AC is turned on.

I am uncertain of the current rating of these devices. I have run them with 100 Watt lamps for years, so I assume they can easily handle 1 Amp.

As a side note I have found that these devices generate a considerable amount of RF noise. I am a ham operator and always know when the outside lights are on!

Solid State AC Relays.

Refer to the figure titled "Control of an AC Relay".

These units are turned on with a typical minimum voltage of 3.0 VDC and are capable of switching either 120 or 240 VAC at typical currents of 10A. The DC input is optically isolated from the AC semiconductor switch. The AC "contact" is normally open. T hat is, when no control voltage is applied, the AC is off.

In the past, these have been quite expensive, but I have found many at hamfests (typically $2.00) and now note that Jameco offers two Teledyne models at less than $15.00. It has been my experience that the minimum and maximum DC values on the control sid e and the voltage and current ratings on the AC side are clearly marked on the unit. Jameco indicates the models they offer may be driven with either CMOS or TTL.

As I am concerned for the safety of my students, I usually wire an assembly as illustrated in the figure titled "Switched 120 VAC Module". The assembly consists of a metal electrical box capable of holding two duplex outlets, and a cover plate. The soli d state AC relay is bolted to one side of the rear of the cover plate, which requires some minor drilling and the duplex outlet to the other side of the rear of the cover plate.

My hesitancy in using plastic electrical boxes is perhaps being cautious, but if there is even one Ohm of resistance associated with the AC contact, including the wiring connections, at 10 A, the power dissipation is 100 Watts on a relatively small surfac e area. I feel more comfortable using a steel box.

Note that the normally open switch associated with the AC relay should be placed in series with the "hot" side of the AC line. Unlike electronics, when working with AC, the hot lead is black and the cold lead is white. However, it is never wise to trust the last person followed this standard, and I usually measure the AC voltages. The voltage from white to ground (green lead) should be less than a volt, and from black to ground, the voltage should be 120 VAC.

Other Uses.

Refer to the figure titled "Use of AC Relays to Remotely Switch Power".

I have eight incandescent lamp fixtures in my basement, on several branch circuits with no central switch, and my family likes to turn lamps on and rarely turn them off. To avoid the nightly ritual of walking down into the basement and going from lamp to lamp, I wanted switches at the top of the stairs. However, installing AC switches in the AC line for each lamp appeared a massive undertaking and use of these AC relays made the job quite simple. An AC relay was co-located with each lamp fixture and in serted in the "hot" side of the AC line. However, the other wiring, was a simple matter of running the DC pairs to the top of the stairs and using eight miniature toggle switches mounted on an aluminum panel.