Reading a Anemometer reed switch with a 555 timer

Hi!
I have an Anemometer (3-cup wind sensor) at the side of my house for reading wind speeds. When the wind blows the cups turn which operates a reed switch inside the unit. Each turn gives 2 pulses from the reed switch.
I ran the reed switch contacts through an opto-isolator. The other side of the opto-isolator is linked to an I/O card on my PC for wind monitoring (obviously!).
The reed switch had about 5 years of operation according to the anemometer manufacturers.
Unfortunately I have 'welded' the contacts on the reed switch after only one month! It seems that connecting the reed switch in series with the LED inside the opto-isolator has passed too much current through the reed switch.
What I am after is a component that can trigger a higher load (it's only an LED!) from a extremely low voltage/current input. The reed switch will then be switching a 'next to nothing' voltage and therefore last longer!
The reed switch is firing about 5 times each second, so the lower the voltage going through it the better.
I am thinking of using this circuit:
http://www.kpsec.freeuk.com/555timer.htm#buffer
How do I supply a low input voltage to this circuit?
If anyone has any ideas or recommendations that would be excellent!
Thank you very much,
Matt

Posted by Matt on 22/09/2007, at 01:39 GMT

Include a resistor

A reed switch should not be damaged by the small current required for an opto-isolator LED. I think you may have omitted a current-limiting resistor in series with the LED and this means a high current will pass, risking damage to both the reed switch and LED.

Connect a resistor in series with the LED to limit the current to about 10mA (or as recommended on the datasheet for your opto-isolator). 1k ohm should be suitable, the exact value is not critical but this is the formula:

Resistance in ohm = (supply voltage - 2V)/10mA

Posted by John Hewes on 24/09/2007, at 14:49 GMT

Hi John

Actually I do have a resistor in series. I even placed the highest resistor possible in order to reduce current going through the reed switch. I ended up with a 4k7.
I have been designing projects since I was very young (35 now).

So from what you are saying about the LED should not be able to burn out the reed, the opto-isolator could be drawing a high current for some reason (may be a fault on the PCB - it's one that I constructed, may be there is a linked track). Other than that, well I don't know! To stay on the safe side, I'm going to construct an RS flip flop circuit.

I have noticed the resistors on other opto-isolators getting warm (1/4 watt resistor) when they are on. They are all individual units, so may be it is the opto-isolator!

Thanks anyway. Matt

Posted by Anonymous on 24/09/2007, at 21:30 GMT