@▷ LDR Pole Light Switch | Diagram for Schematic

LDR Pole Light Switch

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 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.

Pole Light Switch Circuit Schematic

LDR Pole Light Switch Schematic

Power supply

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.

555 Schmitt trigger driver

A TLC 555 was used as a voltage threshold detecting device with hysteresis. Pin 7 drives the TRIAC gate directly via its open collector output. This is a rather unconventional application. The CMOS version is used to minimize power supply load—I tried a bipolar 555 and it worked, but the power supply ripple voltage doubled to about 0.5VP-P.

Constant current bias—Threshold adjustment

Because the 555 has so much hysteresis, I feared that the ON & OFF thresholds would be too far apart. To help reduce the hysteresis, the LDR is biased by a current source. Q1 is wired as a current source—its collector current does not vary with collector voltage. This technique essentially increases the “gain” of the LDR. The current is set via adjusting the emitter resistor (R3)—it drops about 0.37V.

C3 makes the circuit insensitive to rapid changes in light intensity. It takes about 60sec to turn on.


The logic TRIAC is an interesting device. It can be triggered by either a positive or negative gate current regardless of voltage blocking polarity. For maximum sensitivity, I used negative gate current. The device I used had an actual Igt (gate current sensitivity) of 1.5mA that is well below the 5mA Max specification. However, gate overdrive (5mA in my case) is recommended to assure that it fires at low winter temperatures. Quencharc RC-1 is connected across the TRIAC to help control turn-off voltage transients.

Choice of lighting

For the time being, I am sticking with the vintage incandescent lamp—it is a matter of aesthetics. I will upgrade to LED technology only when its color balance matches incandescent.

Photo gallery

For the future

TRIAC phase control

Glossary of undocumented words and idioms (for our ESL friends)

tweaking –small adjustment (electronics)

to boot –idiom—literally, to kick around—In addition to—fringe benefit—“extra spending money to boot”

time being –idiom—the present time, presently, for now…

Preferred components for the serious experimenter

2N6075B, Logic TRIAC, 4A, 600V, Igt = 3mA, TO-225AA
DigiKey 2N6075BGOS-ND, $0.69 each

LDR (CdS Photocell), Advanced Photonics PDV-P8103
DigiKey PDV-P8103-ND, $0.80 each