Relay Voltage Selection

kmitchl

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I have completed two cameras to date. The first was powered from a 12 volt sealed lead acid battery and used a 12 volt relay in the Mainstay MW2 PIR. Works fine.

On the second camera I used three 9 volt batteries in parallel and a 9 volt relay. I could get an indication from the walk LED but could not get it to trip the camera.

I found that I needed about an additional 1.2 volts to trip the PIR relay. From tracing the circuit I found there is a transistor which is switched by the IC in the PIR. When it's fully on there is still about a 1.2 volt drop from the collector to emitter. This in series with the relay coil was enough to keep the relay from operating.  

The options were to use a 6volt relay or increase the supply voltage by 1.5 volts. I added a single "C" cell in series with the 9 volt batteries. On any more cameras I build I'll use a 6 volt relay.

I think the reason the 12 volt model worked was the voltage although reduced is still within the tolerance of the pickup voltage of the relay. Using the lower voltage relay coil has the added advantage that it will continue to operate as the battery is discharged and the output voltage decays.
 

Archilochus

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Hi km,
Just looked over Tinhorns MS-20 schematic.  The resistor (R28) going to the base of the relay driver transistor is of a fairly large value (100K).  Don't know the specs for that transistor, but you might try a smaller value resistor at the base (say 22K or 47K) - would probably "saturate" the transistor and get it to turn fully "ON"

The combination of 9 volt batts and one "C" cell could lead to some odd behaviour. The "C" cell has *much* greater per cell capacity.  I think I'd go with the 6 volt relay if lowering the value of the base resistor does not help.

Good Luck,
Archilochus
 

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