Hi all

Yet another 555 relay question, though I hope this one is different enough to make a worthwhile post. Disclaimer: I am an electronics nuff-nuff, I own a soldering iron and can make an LED flasher or a siren for my kids bike but that is about it so please use small words if you reply.

The ultimate aim is to close a solenoid on an air compressor valve, 12vDC at about 600mA. The solenoid only needs to fire for about 500ms, just enough to shoot a small burst of air through the valve then shut off again until I press a button once more.

My intentions are to:

• Use a 555 wired as monostable to give me a pulse I need. I think I am OK on this part using 1.1xRxC to give me the right pulse time and feeding this circuit from a 9v battery. Input would be from a SPST micro switch.
• Use the pulse to energize the relay coil. Am I right that I need to pick a relay that has a high enough coil impedance to not exceed 200mA for the 555 ?
• Provide a separate power feed on the relay contacts to close my solenoid.

I have spent the last three days reading posts here and elsewhere and I think I understand what I am reading but I have also read many times about the perils of inductive loads on 555 chips.

Any information or advice most appreciated particularly on the "downstream" parts of the circuit as to the relay and solenoid and how to protect the 555 circuit from them.

Thanks for reading.

 

So you aren't feeding a relay, but a solenoid? If so the solenoid will be the device that determines the majority of the current.

It isn't very hard. You didn't say what part of the world you are, so I'm assuming the USA. You'll note the old hands have their place of origin on the upper right hand corner of their posts, this helps when recommending parts.

This is very close to what you're looking for...

A relay and a solenoid are close, and for the parts use Ct = 10µF and Rt = 47KΩ. The diode absorbs the surges from the coil, as to the filter caps on the power supply. R3 may be eliminated, or replaced with different value, it helps Q1 from getting hot. If you eliminate R3 and Q1 does heat up then insert a value, such as 10Ω

 

Thanks for the quick reply Bill, much appreciated.
I updated my profile as suggested; current location a very cold workshop in suburban Melbourne, Australia.

I am using the relay as the current draw on the solenoid is heavy (compared to the rest of the circuit) and the proposed timer circuit voltage is much lower than that of the solenoid, does that sound reasonable ?

If that is the case do I still need the MOSFET or will the 555 alone handle the relay coil, with the diode added to damp the back current ?

Cheers

 

OK, you need to tell me the voltage the timer is operating at vs. the solenoid. I was assuming 12VDC throughout. There are much better MOSFETs that is suggested, it was an example.

 

9v proposed for the timer circuit, probably from a PP3 type, though I am open to suggestions, I just want to keep the footprint small for the timer.

The solenoid is 12v, pulling about 600mA from a plug pack.

Thanks

 

Can you up the voltage to the 555 to 12VDC? The MOSFET requires it. You can use a different kind of transistor, but the MOSFET is much better overall, better conductance, runs cooler (no heatsinking required).

If I need to redraw the schematic using a different transistor I will, but you will be much happier with this design.

 

Opps, I was checking my blog out with my alter ego (and there were problems). QuietMan is me.

 

Dang if I didn't do it again.

 

Can you up the voltage to the 555 to 12VDC? The MOSFET requires it.

Sure, no real reason I can't.

 

Sure, no real reason I can't.

If you have an problems feel free to ask, I'm a staple around here.

BTW, the diode could use a 1N4001 or better.

 

Hi Bill

I got it working thanks to your help, I finished up with an IR540 MOSFET as that was all my local supplier could manage.

I added a 100k pot as Rt so I could play around with the values and a 5k resistor in series to stop the pot going to zero.

The circuit works well at relatively high resistance values, 50k and above.
If I switch the resistor to a low value to get a shorter time, anything below about 15k the circuit acts like a straight out switch, as soon as long as I press the switch the relay stays energised.

Any thoughts on this behaviour ?

Thanks

 

Sure, you have the timing set for milliseconds, or something close. There are lots of electronics applications for durations that short are useful. The 555 can time from millionths of a second to an hour or so.