Dark Detector - On for Set Time Then Reset

Thread Starter

uknn

Joined May 6, 2013
13
Good Evening All.

I need to build a circuit that will activate a solonoid (on a water valve to water the plants while I'm on holiday!) at dusk, and remain on for a adjustable pre-set time.

I've come up with the attached design - LDR and comparator trip a 555 flip flop (when it gets dark) to turn on the output transistor and a 555 timer. The timer pulses every 30 seconds to a 4017 which drives an LED indication of time elapsed.

The 4017 outputs also feed via selector switch a transistor which will reset the flip flop and turn off the water. This theory seems sound to me, but before I expend the effort of building it, I'd appreciate any suggestions from the forum to improve it, or if you can see some obvious mistake I've made!

Thanks and Best Regards,
Ian.
 

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Thread Starter

uknn

Joined May 6, 2013
13
I know, but I get a lot more satisfaction from building it myself. :)

And thinking about they already sell tomatoes in the store, so no point growing my own then I wouldn't need to water anything either...... :)
 
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ScottWang

Joined Aug 23, 2012
6,860
If you don't want to use any feedback reset for CD4017, then the reset pin 15 must be connected to GND, and Cout Pin12 should be open, no any connection, it is an output pin you can't connected to GND
 
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Thread Starter

uknn

Joined May 6, 2013
13
Thank you for spotting that Scott, yes I'd grounded the wrong pin by mistake. Just the sort of silly error I hoped you good people would spot before I etched a board for it!
 

Thread Starter

uknn

Joined May 6, 2013
13
Right folks, I've built this thing as per the diagram, but with the correction to to 4017's pins as per Scott's post above. However it dosen't work!

Comparator works fine, but neither it nor the buttons trip the 555 flip flop. If I ground pin 2 or 4 with a probe it flips fine, but it seems the decoupling capacitors are not taking the voltage on pins 2/4 low enough to trip them - should I raise the value of the 100K's after the capacitors, or perhaps change the value of the caps themselves?

Any advice much appreciated!
 

Thread Starter

uknn

Joined May 6, 2013
13
Thanks for the reply Scott, can you explain how reducing the value of the pull up after the capacitor will help? I would have thought that that would just recharge the capacitor quicker - my thinkng is that the 555 pins are not seeing enough of fall off voltage when the capacitor discharges?

Of course I could be completly wrong?
 

Thread Starter

uknn

Joined May 6, 2013
13
Thanks for your time on this Scott, much apreciated, I'll swap the resistors to the 22K's you suggest, and let you know what happens!

Many Thanks,
Ian.
 

Thread Starter

uknn

Joined May 6, 2013
13
Yes, just noticed the "Move" - No wonder it dosen't work - Of course I've etched the PCB wrong, but I luckily that end of the resistor is right next to a +ve track so it's not too much of a problem to correct it.

Many thanks again Scott!
 
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