Relay, Dark Activated

Dark Activated Relay
Parts List:
Resistors are 1/4 Watt, 5%
R1,R3 = 1K5 R2 = 6K8 R4 = 3K3 Rx = 1K-10K (see text. I used 3K3) P1 = 20K, trimmer pot LDR1 = Light Depending Resistor Q1,Q2 = PN200, 2N3906, NTE159, 2N4403, etc. Q3 = 2N3053, 2N2219, 2N699, NTE128, etc. D1 = 1N4001 Ry1 = Relay, 9-12V
This circuit switches a relay at dark via a Light Dependent Resistor or LDR for short.
The applications are numerous. In my application, it switches on a light with an older type movement sensor and is mounted on the top corner of my workshop with the LDR facing upwards. The Dark Activated Relay prevents that the light is triggered during daylight hours. But, it can also be used to switch led-beacon lights for your driveway, or whatever.

The LDR, Rx, and P1 are connected in series to accommodate different ambient/dark levels. Q1 and Q2 are wired as a trigger flip-flop; when Q1 is on, Q2 is off and vice-versa. R1 gives some positive feedback to the emitters of Q1 and Q2. The resistance of the LDR during daylight is much higher than the 20K trimmer and Rx and will produce a voltage at Q1 higher than the base voltage of Q2 which will activate the relay. The type I used myself is a Potter&Brumfield relay, 12V/10A, model T7NS5D1-12, and cost only a couple bucks. As a power source I use a 12 volt/500mA adapter.

For Rx I used a 3K3 resistor to accommodate the LDR type I'm using. It may not be needed for your own scenario and in that case use a 1K resistor for Rx. Trim pot P1 will ajust the 'dark' sensitivity.
The current draw during off-state (daylight) is about 3mA standby-current, draw during dark is only 130mA, depending on your selection of Ry1, and semiconductors.

I will make a printed circuit board and layout sometime later this year.

Back to Circuits page
Copyright © 1995 -- Tony van Roon
Last updated January 23, 2009