I am posting a circuit that appears to be an odd 555 application.It is a controller board for a Chinese made infrared floodlight. The board takes the output of a Light Defined Resistor on the floodlight’s LED board and switches the light on/off depending upon ambient light conditions.
The light worked great for a couple of weeks and then quit working. I isolated the problem to a little circuit board within the lamp’s waterproof metal housing. I suspected that the heatsinked TI-220 transistor was bad. I expected it to be a MOS-FET, which I have in stock, but under close examination it turned out to be a PNP Darlington transistor, which I do not have on hand. I noticed the little 555 timer and wondered why it was there, and, of course, it could be what failed.
Instead of using the normal pin-3 output, this circuit uses pin-7 as an open collector switch. I haven’t fully deciphered the circuit because (1) I rewired the lamp directly, eliminating the board – just letting the lamp stay on continuously. (2) Since I don’t have a working circuit to scope out I can’t prove any theories of how it works. I suspect that, due to parts availability, using a Darlington and 555 was simply cheaper than using a MOS-FET.
Anyway, the images below show the circuit, as I reverse engineered i,t as well as photos of the board and lamp. If anyone knows how this circuit works I would love to hear about it. The last photo shows the LEDs on but remember that digital cameras can see infrared so these LEDs are not so visible to the naked eye. They emit a pale pink glow to the naked eye but it works like a strong floodlight to an IR sensitive camera. Also, the heatsink was removed from the TO-220 before I took the photos.