Step 9 : Circuit overview
I'm not an electronics expert but am self-taught so if what I say isn't strictly correct, don't worry. The end result is all that matters.
The circuit inside a low voltage 8-function controller (24v or 35v) is something like the following. It varies slightly from one model to another. For 110v or 240v ones it should be similar, normally with the addition of a large 400v greencap capacitor to cut down the power.
The circuit consists of:
- A section to convert the AC to DC and drop it to around 5v.
- A controller IC which is either a 14-pin DIL (sometimes marked LS-ll) or one of those blobs of black plastic on a small vertical daughter board.
- Four SCR's or Triacs (the things that look like transistors) to turn the lights on and off.
- The push-button (that grounds one pin of the IC to change functions).
(NB. the push button may not ground the IC pin completely. Sometimes the bottom of the rubber button is resistive so when pressed it may insert say 100 ohms between the IC pin and ground. So when testing make sure you use the rubber button, don't just short out the grid under the button with a screwdriver etc. or you could damage the IC.)
In more detail, the AC input goes through a bridge rectifier, normally consisting of 4 discrete diodes, to rectify it (make all the AC pulses go in the same direction). It then goes through a dropping resistor, (30k for 35v controllers) to reduce the voltage and then into the IC +ve pin. There is a 100uF cap at the IC +ve pin to turn the rectified AC pulses into relatively flat DC.
If you turn off the controller briefly then turn it back on again it will remember the function setting because the 100uF capacitor keeps up enough power to the IC. But over time it cannot maintain sufficient power. The circuit modification involves increasing the charge so power is maintained to the IC. All the power it needs is the smell of some electrons on an oily rag (From memory about 7uA at 1v).
Depending on the model of controller, 1000uF may be sufficient to run the chip for the required 20 or so hours each day while the power is off. Other models may need more charge, such as a 1F, 5V memory backup cap. I got lucky and recovered about 20 of those from some discarded circuit boards. Failing that they cost about $5 (or they did until the local supplier stopped supplying them.)
