How to modify a motion sensor circuit so it only turns on a light at night

Thread Starter

Larrycruzen

Joined Oct 1, 2019
11
I have a PIR motion sensor schematic that feeds a relay to turn on a device like a 110V light. I’d like to modify it to include a photoresistor (LDR) so it only turns on at night.
I expect this an extremely common application, but haven’t been able to find such a schematic. Anyone got one to recommend?
 

TeeKay6

Joined Apr 20, 2019
381
I have a PIR motion sensor schematic that feeds a relay to turn on a device like a 110V light. I’d like to modify it to include a photoresistor (LDR) so it only turns on at night.
I expect this an extremely common application, but haven’t been able to find such a schematic. Anyone got one to recommend?
Post your existing schematic so that we can consider it.
 

djsfantasi

Joined Apr 11, 2010
5,585
Or with both a photo resistor circuit, that outputs a high voltage level when it was dark, and a PIR circuit that outputs a high voltage level when motion is detected, then you could create a transistor AND circuit that only outputs the desired signal when BOTH inputs are high.

Design the three sub circuits separately and combine them to get your desired result.

I’d post a schematic, but you need to supply more information. First, the output of the PIR. Then, the photosensitive circuit can be designed. Finally, the AND circuit needs to be designed based on the outputs of the two sensor circuits.

Google the sub circuits separately and you’ll get several suggestions. Then Google “transistor AND circuit” to find schematics for the logic.

This is a common approach to design a circuit. If all you’re looking for is a circuit for the final result, you may not find what you’re looking for. By breaking the function in sub circuits, you are much more likely to have success.
 

Thread Starter

Larrycruzen

Joined Oct 1, 2019
11
Thank you, gentlemen, for the prompt replies. I didn’t post any circuits or pictures because I didn’t want to violate any rules of this forum by posting other people’s circuits. I’ll try to give credits for the following data.

2B1388D4-3F28-47ED-A53D-32047BFA1D9B.jpeg83F055E5-4E31-40DB-A642-8A7A71954165.jpeg
My PIR is a common HC-SR501. My relays are 5V trigger, 250V/10A max output. Haven’t yet measured the trigger current, but I’d guess it’s about 50 milliamps. My LDR measures 6 megs dark and 400 ohms sunlight. I’d like it to switch the BC 557 at about 22K ambient light. I will adjust both circuits to operate at a common voltage of 4.5V (3 AA batteries) so I don’t want the circuit to be a current hog.
First time I’ve tried to post attachments. Hope the thumbnails are viewable.
This whole exercise is to try to self educate an old, old mechanical engineer, from the vacuum tube age, in the practical application of some of these infernal contraptions of today. If successful, though, I plan to make several units to control night lights around the house.
 

Thread Starter

Larrycruzen

Joined Oct 1, 2019
11
Looks like using the LDR circuit to switch power to the PIR circuit, as suggested by Mr. Crutschow, would be the way to go.
 

Alec_t

Joined Sep 17, 2013
10,355
Be aware that your LDR circuit currently has no hysteresis, so the slightest change in light level around the trip point will cause the output to dither between on and off and result in the relay chattering. A circuit using a comparator with feedback to provide hysteresis would be better. There are many out there on the 'net.
 
Thank you, gentlemen, for the prompt replies. I didn’t post any circuits or pictures because I didn’t want to violate any rules of this forum by posting other people’s circuits. I’ll try to give credits for the following data.

View attachment 190854View attachment 190855
My PIR is a common HC-SR501. My relays are 5V trigger, 250V/10A max output. Haven’t yet measured the trigger current, but I’d guess it’s about 50 milliamps. My LDR measures 6 megs dark and 400 ohms sunlight. I’d like it to switch the BC 557 at about 22K ambient light. I will adjust both circuits to operate at a common voltage of 4.5V (3 AA batteries) so I don’t want the circuit to be a current hog.
First time I’ve tried to post attachments. Hope the thumbnails are viewable.
This whole exercise is to try to self educate an old, old mechanical engineer, from the vacuum tube age, in the practical application of some of these infernal contraptions of today. If successful, though, I plan to make several units to control night lights around the house.
I am not aware of any rules about posting circuits from other authors. I see manuals and product schematics all the time. For this circuit as posted you could connect the LDR between the base and the emitter of the transistor. When illuminated the LDR will prevent the transistor from switching on, when it is dark the much higher resistance will have much less effect. That is about as simple as it can be.
 

djsfantasi

Joined Apr 11, 2010
5,585
Thank you, gentlemen, for the prompt replies. I didn’t post any circuits or pictures because I didn’t want to violate any rules of this forum by posting other people’s circuits. I’ll try to give credits for the following data.
I think that what you want to post falls under fair use. If you were to post someone else’s circuit for personal monetary gain, then... But in a post asking for assistance, you’re good. Attributions are always helpful, but not necessary.
 
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