Adding a delay to a 12vdc circuit

Thread Starter


Joined Nov 12, 2020
Hello folks. I lack a great deal of knowledge in this area, but have cobbled together something of a solution to automatically open and close the door of our henhouse at dawn and dusk. Here's a rough diagram.

The setup comprises a small 12v battery, a solar panel to charge it, a retail dawn-dusk light sensitive switch, a DPDT relay and a linear actuator.

The problem I have is the LDR switch closes a few lumens too soon of an evening. I chose the retail device as it's housed in a moulded IP68 housing, but the downside is I can't open it up to modify it.

I'd like to be able to delay the signal to the DPDT relay from the LDR switch by maybe five minutes, and my hazy recollection of electronics from school suggests I could possibly use a resistor and cap between the LDR switch and the relay, but quite how I have no idea.

Can anyone suggest a simple solution here?



Joined Aug 1, 2013
Your memory is correct in concept - an R-C circuit can have the effect of delaying a transition from one voltage level to another. But ...

We need to know the part number of the relay or its coil characteristics (voltage, current, power, etc.), but even without that information it is fair to say that a multi-minute R-C delay that delivers enough current for a relay coil would need a massive capacitor.

Quickie example: 12 V relay, nominal coil current is 0.1 A, guaranteed turn on at 75% (9 V), delay period is 5 minutes (600 seconds):

R = 30 ohms
C = 20 farads (that's a lot)

Way better to buy or build a timer circuit; ebay has many.

Just to clarify - s

Sun comes up, solar cell energizes relay coil, actuator moves, relay coil stays energized all day

Sun goes down, solar cell voltage drops, relay coil drops out, battery runs actuator motor

Is that the process?


Thread Starter


Joined Nov 12, 2020
Thanks very much. Yes, if I interpret 'solar cell' to mean not the PV panel but the LDR switch. So the battery powers the LDR switch continually, the PV cell charges the battery when it can. The switch then energises the relay at dusk and off at dawn, but dusk is a bit too soon/chickens are a bit too stupid to run for the door when they hear the motor whine :)

The relay is an LY2NJ. Off to read about timer circuits now... I thought about building something on top of a nearby RPi which already controls a brewing fridge, but feel this is over-complicated and doesn't follow the mantra of 'adequate complexity'.

Thanks again for your help.

-edit- I've also been thinking more laterally around maybe just angling a small west facing mirror at the sensor in the hope of doubling the light intensity that it 'sees'. Will test that approach this eve.


Joined Mar 10, 2019
There are lots of timer relays on places like Ebay, like the H3Y-2 series made by Omron, and can be found with 12VDC input and maximum delays from seconds up to 60 minutes. They are adjustable. So, if you get a 12VDC, 10M delay timer, you can adjust it to 4,5 or 6 minutes depending on exactly when you want it to trigger.