555 load question

[hiya]

hi,

Have a project that requires activating a 12dc 9w solinoid and was wondering if this circuit will work ok or if there might be a better way.


I am replacing a dozen urinal flush sensors with push buttons. Yeah I know it seems a step backward but it turns out a push button is the most water conservative device available and we are on a bit of a water saving trend here.

The current system has a 1.5 amp 16vac plug pack going into a black box containing pretty flasing lights and a microwave sensor, this in turn connects to a 12dc 9w solinoid on the water pipe.

I am going to try to rectify the ac, use a LM7812 regulator, a NE555 in monostable mode to drive a BD681 (NPN TO-126 36W 100V 4A 750 1.5A) to open the valve. I need an ajustable time from 5-20 seconds no matter how slow/fast the button is pushed.

Thanks

 

[Russlk]

I could not find data on the transistor, but I assume it was not a darlington. This 555 circuit should work:

 

[electronist]

Hello hiya,

Your ckt seems to be allright but few things should be noted

instead of connecting the transistor directly to the load it would be preferable to use a cheaper transistor like BC547 in switch mode and trigger a relay at the output.
This will act as a buffer to the ckt and maybe also reduce the cost and size of the designed ckts.
PCB mountable relays are easily available that will make your ckt compact.
Also your ckt seems to look fine. Instead of connecting a voltage divider at the base of the transister you can connect a 10K resistor directly between pin3 of 555 and the base of the transistor (BC4444547 i.e. ) . Transistors are current controlled and not voltage controlled.
The speed of pressing the button wont count in a monostable as it is a one shot ckt.

 

[Roff]

If the solenoid is truly a DC-only solenoid, and was already in place, why would you need a regulator for it? I can see why you might need a regulator for the 555, but for the solenoid... ?

 

[hiya]

electronist,

I didn't want to use a relay as I will need to build 14 units if it works. A transistor would be cheaper in the long run.

Ron,

The only things I will be using is the 16vac plug pack and the 12vdc 9w solinoid. The control/sensor/power supply is one unit and will not be used. I guess if the solinoid uses to much current I can connect directly to the V+ of the bridge.