I have an Owl that turns it's head about once per minute when the sun shines. The circuit in it died and I want to make a replacement. It's an interesting challenge because there is no battery, just a 3V solar panel and 10,000 ufd capacitor to activate a tiny solenoid about once a minute. Here is the basic original operation:

It starts with the big cap at 0V
The sun begins to charge the cap
After about 1 minute, the solenoid fires
The solenoid firing discharge the big cap to near zero and the process starts again.
The first cycle was longer than a minute, but subsequent cycles are somewhere around 1 minute each while the sun shines​

I have tried a couple different ways to accomplish this, but so far nothing works. Basically I was trying to do this with an RC circuit driving a MOSFET which drives the solenoid.

Ideas for how to do this are welcome

 

First. A picture of the capacitor and surrounding parts would help. There has to be at least one more element you haven't mentioned as the solenoid would not work with just the three parts you listed

 

There has to be a "breakover" negative-resistance element like a Diac or an old unijunction transistor somewhere. Basically as the capacitor charges there can be no conductive path to the motor other-wise the capacitor wouldn't charge. Only after a certain voltage is reached would the diac conduct, dumping the charge in capacitor into the solenoid.

 

First. A picture of the capacitor and surrounding parts would help. There has to be at least one more element you haven't mentioned as the solenoid would not work with just the three parts you listed

Yes, you are correct. The missing part is the circuit I am trying to design. A picture of the circuit board isn't going to help because it's sealed.

 

Could it be something as simple as a 3 volt zener in series with the solenoid?

 

Any idea what the open-circuit voltage of the solar panel is?

ps. I get to fly my avatar any time I want; do you get to fly yours?

 

Could it be something as simple as a 3 volt zener in series with the solenoid?

no.

 

A zener won't do it, but a unijunction transistor (or a simulated one using an NPN and a PNP) ought to.

 

A zener won't do it, but a unijunction transistor (or a simulated one using an NPN and a PNP) ought to.

With the simulated programmable unijunction, you can set the breakover voltage to almost any voltage you want with a low voltage Zener.

Just put the Zener across B/C of either one of the transistors.

 

Here's what I am going to try next,

Any idea what the open-circuit voltage of the solar panel is?

ps. I get to fly my avatar any time I want; do you get to fly yours?

YUP!

 

Here is what I am going to try next.

 

Here is what I am going to try next.View attachment 90287

Most TO220 style MOSFETs need about 6 - 8V on the gate to turn on.

AFAICR: some of the TO92 types like 2N7000 and BS170 will switch at 3V or less.

 

BTS141 switches at 1.3 to 2.2

I think that would work

 

BTS141 switches at 1.3 to 2.2

I think that would work

No need to shout!!!

 

Sorry, I didn't mean to shout! I copied the BTS141 from a web page and it was large bold. When I typed after it, the format stayed.

 

Sorry, I didn't mean to shout! I copied the BTS141 from a web page and it was large bold. When I typed after it, the format stayed.

Don't forget to add up all the volt drops between the capacitor and gate.

Someone else suggested the 2 transistor implementation of the programmable unijunction - I still think you'd do better with that.

 

I think I'll research a PUJT solution, perhaps triggering an SCR.

Thanks for all the suggestions.

 

I think I'll research a PUJT solution, perhaps triggering an SCR.

Thanks for all the suggestions.

The transistor implementation can potentially be an SCR.

The programmable unijunction is basically an SCR with the gate at the anode end instead of the cathode end.

The transistor job is actually equivalent to a silicon controlled switch (SCS) which has both gates brought out on pins.

 

I have an idea, but I need to know the following:

What is the open-circuit voltage of the solar panel?
What is the resistance of the solenoid?

 

In bright sun the open circuit solar cell voltage is 5.94V. The dc resistance of the solenoid is 3.4 ohms.