DIYbanter - View Single Post - Automatic lamp post; lights on at dusk, off at dawn

Rebel1

On 3/25/2015 5:39 PM, wrote:
On Wednesday, March 25, 2015 at 5:05:12 PM UTC-4, Rebel1 wrote:
The circuit is simply three-components across the 24VAC supply: a
two-wire photocell and two 12V (1156 auto bulbs) in series. One would
expect that the resistance of the photocell is close to zero in the
dark, to apply the full voltage across the bulbs, and highest in
daylight to turn them off.

My actual measurements on the cell alone are opposite: about 10,000 ohms
in bright light and close to infinity in the dark. Thinking the cell was
bad, I ordered a replacement; same oddball measurements.

What am I missing?

A photocell by itself is not normally able to turn a light on and off.

A photocell is usually connected to a circuit with a transistor or two and a relay or Triac or something to go with it.

The photocell controls the relay which controls the light.

Mark

I can't visualize how that would work. There are only two wires into the
photo cell. Maybe with the higher 24 VAC applied, something changes to
lower the resistance across the cell, but then 24 VAC wouldn't be
applied anymore across the cell.

A couple hours ago, I sent the question to the company that makes the
cell. I'll post the reply.

Thanks.

R1