Relay driver circuit with timer

[Ejaz]

I need to drive a miniature relay with maximum coil voltage of 5 V and minimum coil current of 200 mA by a timer circuit. The circuit need to turn it ON & OFF every 8-10 minutes. Please help! Thanks in advance!

 

[BrianG]

Have a look at the industry standard 555 or dual 556 timer circuits. These can just about drive your relay directly at 5V but might need an additional transistor to beef up the output drive.

 

[Ejaz]

Using a 555 IC for a 10 minute ON/OFF timer requires a large size of capacitor. Isn't there going to be a problem?

 

[BrianG]

Whoops! Yes, you are right - I had not actually done the sums until now. Even with a 5.6 megohm resistor it would need a 100uF capacitor, and device input current and capacitor leakage current would be a real problem.

You could try the TLC555 version from Texas Instruments (and others). This is a CMOS version so has minute input currents to the chip allowing much higher resistors and smaller capacitors to be used for the timing components.

Alternatively how about a digital approach? The CMOS 4000 series CD4060B is a 14-stage binary ripple counter with built-in RC oscillator. You should be able to get a long enough delay from that with the oscdillator running at a few Hz, but it would of course need an additional driver transistor to power the relay.

 

[Foxfur]

Hi Ejaz,
Take a look at Motorola's MC1455P. In fig 26 in the datasheet (

http://www.onsemi.com/pub/Collateral/MC1455-D.PDF

) it has a schematic for a sequential chain of timers. This still may not address your Iout requirement though, as the MC1455 has an abs max sink/source capability of 200mA. You may consider a small form factor SSR with a logic level input. Omega used to have SFF SSR's (about 1in x .75in x .75 in) and probably still do.
An NE558N quad-timer IC may help conserve board real estate and is well suited to sequential timing with long delays. look at the app note here:

http://www.semiconductors.philips.com/acrobat/applicationnotes/AN171.pdf

It should give you some ideas. also, look at:

http://www1.jaycar.com.au/images_uploaded/relaydrv.pdf

It has some schematics and good design practices for driving relays with transistors as well as from timers(BONUS!).
The Philips App Note has a nifty voltage controlled timer with a 50:1 ratio that I am going to have to build for an upcoming project. I'm glad you posted your question as I would not have stumbled across the Philips sheet without your question.
Good luck, you'll do just fine !
Foxfur

 

[buzzp]

What are you using for a power supply, batteries or line voltage? If using line voltage, consider using the known frequency for timing purposes.