I'm designing a circuit that will open a chicken coop when the sun comes up. I've attached a bungee cord to the door which opens the door when a pin is released. This pin is released by a car door actuator: http://amzn.eu/9r2gjnP

To power the actuator I've picked up a solar panel: http://amzn.eu/e3XX7WB this doesn't have the current to power the actuator so I'm thinking of pairing it with a rechargeable 9v battery.

I've come up with this but am not an expert by any means so would really appreciate it if someone could sense check it for me? Falstad Simulation

The idea is that the sun coming up triggers the LDR, this triggers the 555 timer which in turn allows power to flow to the actuator for a couple of seconds. I may have overcomplicated it so welcome opinions! :)

UPDATE 10/07/2018 15:00 (BST)

Based on feedback from @Chris Stratton and @brhans

  1. Updated schematic: Falstad Simulation V2
  2. The battery I was looking at: http://amzn.eu/6SgbNqA

UPDATE 13/07/2018 21:13 (BST) - Lower tech solution

So I decided to go with @Drew's suggestion of a lower-tech solution. However, I've run into a problem. My relay is simply making a buzzing noise when I expose the LDR to light. Is there a fundimental mistake in my circuit design? Falstad Simulation V2

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    \$\begingroup\$ What sort of LDR do you have which behaves like a switch, and wouldn't you want to charge your battery form the solar panel? \$\endgroup\$ – brhans Jul 9 '18 at 21:26
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    \$\begingroup\$ You probably want something more like a "gel cell" for this. Also you need to exchange the positions of your transistor and actuator, and add a reversed catch diode across the actuator. Realistically most people would use a low end MCU not a 555. \$\endgroup\$ – Chris Stratton Jul 9 '18 at 21:34
  • \$\begingroup\$ It's just a normal LDR, I just used a switch in the simulation to simulate it being fully lit / unlit. Is that an unreliable method? Yes I do intend to charge the battery, good spot. I'll update the diagram. I'd rather not have a massive battery, and the actuator works well on a 9v supply. What would the advantage be? I've never used an MCU and have tonnes of 555s :D \$\endgroup\$ – Dan Jul 10 '18 at 8:44
  • \$\begingroup\$ UPDATED SIMULATION = tinyurl.com/yd6l573s \$\endgroup\$ – Dan Jul 10 '18 at 8:51
  • \$\begingroup\$ Looking at this battery amzn.eu/6SgbNqA \$\endgroup\$ – Dan Jul 10 '18 at 13:57

Based on Update 2...

If the relay is buzzing, then the chances are that the battery you're using can't supply enough current.

The Sun gets bright enough to turn on the transistor circuit. The relay turns on. The actuator turns on. The voltage from the battery drops. The relay drops out. The battery voltage rises again. The relay turns on. The actuator turns on. The voltage from the battery drops. The relay drops out. And so on...

  • \$\begingroup\$ Ah of course, makes perfect sense! I'm waiting for a much better battery to be delivered (hopefully tomorrow) I'll see if that solves it. If not I'll change to a 2 battery design. Thanks! \$\endgroup\$ – Dan Jul 13 '18 at 22:28
  • \$\begingroup\$ @Dan Another concern I have with that simple circuit is that the Sun doesn't rise suddenly in the morning. Instead, the light level gradually increases. So the transistor will spend a significant time half-on. This will tend to drain the battery. It might also overheat the transistor (depending on what transistor you've used and any heatsink you've added). \$\endgroup\$ – Simon B Jul 14 '18 at 13:32
  • \$\begingroup\$ Good point, I'll have a rethink! \$\endgroup\$ – Dan Jul 14 '18 at 16:09

Honestly this day in age, I would just use an arduino. I would also replace that BJT with a mosfet, or just use an arduino-ready relay board.

If you can find some kind of power bank with a built in solar panel that would take care of the solar and charge controller in one shot.

Those door actuators can draw 2 amps or more at 12v so make sure your supply can handle that. I find that adding a 2ohm series resistor often helps to limit the max current.

Also, I only googled it briefly, but I would not connect your solar panel to battery like that, it could overcharge the battery. I think you need a charge controller, or at least some kind of regulator.

There's a goofy low-tech approach you could consider. Just hook the solar panel straight to a relay coil. Put the battery and actuator on the NO contacts of the relay. That way when the sun gets bright enough, the relay contacts will close and the actuator will fire. To keep the actuator from staying on and draining the battery, just put a switch in series that opens when the door opens.

Edit for update 2:

That actuator will want to draw 1A or more. At 12v it will draw more than 2A. You'll need a bigger battery, like a lead acid or 2 lantern batteries in series.

I would wire it like this:


simulate this circuit – Schematic created using CircuitLab

Another way you could do it, which is a little more complicated but which may switch on the relay more cleanly is this:


simulate this circuit

Basically it provides a path for the lantern batteries to power the relay briefly. I requires a couple of extra diodes.

  • \$\begingroup\$ Ha there's something I quite like about the simplicity of the goofy solution! Will have a think \$\endgroup\$ – Dan Jul 11 '18 at 5:22
  • \$\begingroup\$ Diagram for the "goofy" approach :) tinyurl.com/y7zmwvkh I think going more complicated than this is overkill. If I were going to do "open at this time" etc an arduino would be more useful but no reason not to KISS here. \$\endgroup\$ – Dan Jul 11 '18 at 8:15
  • \$\begingroup\$ I'm having some problems with the relay in the "goofy" approach. I've updated the initial description and would appreciate any advice. \$\endgroup\$ – Dan Jul 13 '18 at 20:27
  • \$\begingroup\$ Thanks Drew, I've taken the solar panel out as even though it would produce anything between 6-12v it didn't produce enough current to power the relay. I tested it by connecting it straight to the relay. I know the panel works as it can power an led. \$\endgroup\$ – Dan Jul 14 '18 at 5:24
  • \$\begingroup\$ Bigger solar panel? \$\endgroup\$ – Drew Jul 14 '18 at 5:26

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