What is the purpose of this 1K ohm resistor in this dark sensor circuit?

wayneh

Joined Sep 9, 2010
17,153
It divides down whatever voltage is output from the op amp by 1/11. So the voltage applied to the base of Q1 ranges from 0-1.2V. Sort of. The 741 cannot get near either power rail, so it's really more like 0.3-0.9V.

The capacitor kills the chatter, as Bertus beat me to noting. You can see that the transistor can be turned off faster than it can turn on. It can charge only through R4 but is discharged through both R4 and R5.
 

AnalogKid

Joined Aug 1, 2013
9,358
The resistors R4 and R5 at the output of the opamp with the capacitor C1 will delay the light transients at the input of the light sensor.
This delay will reduce the shatter of the relays.
Don't think so. R4 limits the Q1 base current to a safe value., although I think 10K is too large and could prevent Q1 saturation. 2.2K probably is a better value for that.

However, lowering R4 decreases the R4-C1 time constant. The time constant as shown is 2.2 seconds. If the purpose of this RC filter is to keep a noise burst out of the 741 from causing relay problems, 2.2 seconds actually might not be long enough because the 741 circuit does not have any hysteresis, and the output noise burst could go on for many seconds as the light slooooooowly dims. Separate from that, I think the RC time constant of 2.2 seconds is so slow that it *increases* the possibility of relay contact chatter.

ak
 

wayneh

Joined Sep 9, 2010
17,153
I agree it would be better to filter the sensor and build in hysteresis into the op-amp feedback loop, than to try to use an RC filter on the transistor's base current. The first insures fast switching, the latter guarantees slow-ish switching.
 
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