I am at a road block on my current project. So far I have an AC timer that will trigger 3 times a day and turns on a solenoid and allows flow of a substance onto a resistive pad that varies with weight. Then I have a comparator that has a reference value and a measured value. The "measured" value changes with the resistive pad. It is setup so that when the "measured" value reaches the value of the reference it will trigger a fet and shut off the solenoid. Now this is where I'm stuck. I need this circuit to only operate once. Meaning that I want it to dispense until the "measured" value is met, shut off the solenoid, and then wait until the next time it gets triggered. I feel like there is something very simple that I'm missing but I can't think of it. Let me know what you think and if you need more info. Thanks!

 

You seem to need a feedback path from the "off" part to the "timer" part, but it is impossible to help you without a schematic of what you have accomplished. Can you provide a schematic?

 

Sure, I will see what I can throw together. It will be alittle rough because I haven't set some of the variables in stone yet. Thanks for the reply

 

I tried posting this another way but it hasn't shown up yet.
The timer isn't in the schematic but it is just a cheapy 24 hour AC timer that has a 30 minute minimum on-time. It has a momentary switch inside that I wouldn't mind cracking into. There will also be a 12 volt power supply. Thanks!

 

So, you don't want the dispenser to reload while the dog is eating, right? (That's supposed to be funny.)

This is going to require a bit of pondering. AC comes on and stays on for too long. Fill once and refuse to respond until an "off" of the AC, and then an "on". Right?

 

That's correct

 

The op-amp is always on. The gate is waiting "on". When AC arrives, it produces +12V for the solenoid. The solenoid activated. The dish fills. The brain goes "off". Currnet through the solenoid atcitaves a sensitive coil relay. The relay stops the power to the solenoid and transfers it to its own coil. It stays latched until the AC fails. Then it resumes waiting until AC happens again.

 

Oh. That is simple. Thanks for the help. My cat will thank you.

 

Yer welcome. Say, "Hi" to the cat for me. LOL!

This can be done a dozen different ways. The point is "how to make a latching circuit". Now that you see how it works, you can change it for the most convenient use of the parts you have.

 

So I have ditched the weighing method and made it a more direct approach. I need to spin a motor for a second or less and the switch S1 will take over and keep it spinning until it reaches "home" and disconnects the switch. My problem is that when the timer TMR1 turns on it is a constant output and make it so the output of the 555 is always on. How can I get it to trigger the 555 once or make the 555 only fire once and go to sleep?

 

Never mind. Figured it out. I put a polarized cap inline with the trigger. It only responds to a positive edge.