LDR Pole Light Switch

Posted on Feb 5, 2014

My vintage (62 year old) pole light has always been controlled by a timer ”a source of continual frustration due the requirement of readjustment for the ever-changing seasons. However, after knocking it over (I backed my car into it ), I decided it was time for an update. The circuit consists of a light dependent resistor (LDR), TLC555 (applied

LDR Pole Light Switch
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as Schmitt trigger), and a TRIAC power switch. The complete assembly neatly fits inside the steel pole light tube and the LDR peeks out through a hole in the side. This was really a fun project, and useful to boot. The power supply is the typical capacitor limited charge pump type that is zener regulated at 6. 2V. Due to the lack of isolation, I was careful to identify the return conductor so that the electronics (including the LDR) would not be floating on the hot lead. For safety, most of the testing was done using an isolation transformer. R1 must absorb a high peak power transient current when power is applied, so a carbon comp, ceramic comp, or wire wound resistor is recommended. Maximum DC current available is 16mA. Actual load is about 6mA. My application was 115V, 60hZ. For 230VAC, the components are indicated on the schematic. In this circuit, +6V is the power return lead, and 0V is the electronic circuit common ”to visualize this, one must screw his head on backwards The light dependent resistor (CdS photocell) that I used was in the TO-5 package that is well adapted for poking through a hole and is held in place with silicone rubber. The Clairex CL703M19 LDR that I used is no longer available and I have been unable to locate the specs. The DigiKey PDV-P8103-ND appears to be a reasonable choice, but may require bias current tweaking to set the threshold. A TLC 555 was used as a voltage threshold detecting device...

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