magneticsreed
I need to build a circuit that closes when it detects the passing of a magnetic card.
It should open again when the card passes again.
So I need a simple magnetic reed (like this one). And some kind of switch component that closes when it detects a current pulse from the magnetic reed (right?).
Can someone tell me what kind of component I should use?
1) That depends on the sensitivity of the reed switch and its proximity to the power line.
2a) Yes.
2b) Either locate the reed switch away from the relay coil or align its axis 90 degrees away from the relay coil's axis, or both.
3) It will chatter.
If you're looking for something simple and robust, you may want to consider a current switch; something like:
I think using 2 digital inputs is the way to go, but absent that, you want to build a 2-bit Digital to Analog Converter.
This pic, from http://tabatha.esg.utulsa.edu/digital/pages/r2r-alt.gif, shows a 3 bit converter. You would simly omit the bottom bit, leaving off the 2K resistor at V1 and the 1K resistor attached to it.
You won't get your table, but you will get unique voltages for each bit. For this 2-bit converter, if your switches provide 5V, one bit will add 2.5V to the output, and the other will add 1.25V. So, none on would be zero volts, if the switch attached to the LSB is on and the other off, you'll have 1.25V, if the switch attached to the MSB is on and the other off, you'll have 2.5V, and if both are on you'll have 3.75V
Your issue with this approach will be implementing it with SPST reed switches. You may need an op amp buffer on each one.
Best Answer
You could use reed switch output to toggle a flip-flop such as a 74LV74.
Simply connect the /Q output to the D input and the clock input to the debounce circuit output. /CLR and /PRE must be tied high (along with any other unused inputs from the other half of the chip).
The Q output will go high and low alternately each time the switch opens. It will not start up in any particular state, to do that requires a bit more circuitry and the use of the /CLR or the /PRE input.
Do not substitute a flip-flop that does not have a schmitt trigger clock input or you may get false operation.