I was asked by a friend a while back to create a timer to turn a pool fill value off automatically. Apparently he would turn the fill value on and walk away only to return to water overflowing everywhere. He gave me a few parameters to follow: 10 minute timeout, easy to use, status lights, and reliable. He also said that it would be controlling a standard sprinkler valve and 24VAC would be provided. With these guidelines in place, I began working out the details.
I initially thought about using a 555 to handle the timing and a relay to control the valve, with some AA batteries to power the thing. I later decided that using batteries would be a problem because they would need replacing often. Since 24VAC was provided I made a simple zener based power supply where the zener maintained a constant 5.1v supply and a resistor would drop the remaining voltage and convert it to heat. These aren’t very efficient but they are easy to use and cheaper than a switching power supply. I had to rule out using a simple linear regulator because the input voltage is beyond the limits. The 24VAC input is actually closer to 40VDC due to AC peak voltage and the unloaded nature of the transformer (24VAC * √(2) + 20%). The current available to the rest of the circuit is also quite low with a zener regulator.
Most smaller relays require about 100mA of coil current which would use everything that the zener regulator could put out and then some. The next best solution was a TRIAC or Triode for Alternating Current. These are like an AC switch which turn on with a gate current and allow current to flow in either direction. These are used in many applications where speed control or dimming is needed. The only drawback is that they don’t shut off when the gate current is removed. They turn off when the current through the device also drops to zero – which makes them great for AC use. I decided to use a NTE3047 optoisolated TRIAC driver. That was my first mistake. The datasheet for NTE parts are very lacking but unfortunately, thats usually all I have to choose from unless I place an order from mouser or digi-key. I’ll get back to this problem later.
For more detail: Automatic Water Timer