Nuts & Volts - September 2006

QUESTIONS & ANSWERS

which generates an output voltage. When the push button is released, the capacitor charges through the 47K resistor at a rate of t = RC. When the voltage across the capacitor reaches about 3.0 volts, the gate flips logic and the output goes low.

Now how does this help us with contact bounce? If we select the time constant of the RC network so that it takes more than 20 ms to reach 3.0 volts, any voltage excursions of the contact bounce will have their tops lopped off like a lawnmower over grass. The typical debounce time is anywhere between 50 ms and 0.1 seconds. If the resistor is 47K and the debounce time is 50 ms, then the capacitor is 1 μF (C =t / R).

While this design works well for driving most CMOS devices, TTL and low-voltage logic chips require a cleaner pulse. Often a Schmitt trigger (Figure 1(b)) is used in place of an inverter. Unlike a standard logic gate — which has its high and low trigger voltages at three and two volts, respectively — a Schmitt trigger has a greater hysteresis with trip points at 3. 3 and 1.8 volts. This wider voltage band lets the Schmitt trigger clean up contact bounce that would otherwise pass through a standard gate. Notice the inverse arrangement of R and C — they are in parallel instead of serial. In this arrangement, C charges when S1 is closed and discharges through R when it opens. When both NAND inputs are high, the output goes low.

For really dirty switches, you need to bring out the big guns — one-shot monostable multivibrators. Monostable multivibrators are sometimes called pulse stretchers because once triggered, the output remains high until the one-shot times out. No amount of hammering on the input can disturb this output in its duties. Figure 1(c) and (d) show two monostable designs. Circuit (c) uses a pair of NOR gates to form the one-shot. Circuit (d) isyour classic 555 monostable multivibrator.

A/C FOR A/V BUILD-IN

QI have a multi-media cabinet with a small 120 VAC muffin fan that I would like to control with a temp switch: on at 85°F, off at 80°F. You mentioned a stereo fan cooler in the June ‘02 issue, but didn’t show a schematic. I need a schematic.

— Mike

■ FIGURE 2

115VAC

Fan

180

TRIAC

40C

Thermal Switch

Multi-Media Cooling Fan

AActually, there was a schematic — but it’s not one that you can use because it was a variable speed controller for a 12-volt fan. What you need is a thermal switch, like the R2004025 switch from Cantherm and available from Digi-Key (800-344-4539; www.digikey.com). See Figure 2.

This switch is normally closed (NC) and opens when the temperature inside the cabinet exceeds 104°F ( 40°C). (I know you wanted 85°F, but 104°F is more practical.) Basically, if the cabinet temperature is below that of the bi-metal switch, the contacts remain closed and prevent the triac and fan from turning on. When the temperature inside the cabinet exceeds the switch’s limit, the fan turns on and stays on until the temperature drops significantly.

Why a triac instead of powering the fan from the switch itself? Control. This way, you can up the switched current by simply replacing or paralleling the triac to operate any number or type of fans using the same switch.

feet away for emergency purposes. When doing experiments, sometimes things go wrong that start a fire and I need to be able to turn off the laser FIRST and THEN put out the fire. I don’t want to inadvertently walk into the invisible beam while solving the emergency.

— Bob Slusher

PERIMETER SECURITY

QHaving just finished construction of a CO₂laser, I would now like to know how I could build a simple radio transmitter with a momentary contact switch that would allow me to trip off a 110 VAC switch at 10 or 20

AYipes! I wouldn’t rely on a radio signal to prevent a laser beam from sawing me off at the knees. Instead, I would ring the laser experiment with another laser beam — actually a photoelectric beam that would switch off the 110 VAC if broken.

I would begin with a laser pointer — the kind you can buy for a couple of bucks. Using adjustable mirrors, ring the experiment area with the pointer beam — whose red dot you can see when it strikes a hard surface, like a sheet of white paper — and direct it to a photo detector. See Figure 3.