i need help on comparator

[xiaoxiao]

where can i find a basic comparator circuit and a frequency counter circuit which i use to find the output of a pulse/heart detector as my circuit output is a analog.

thank

 

[pi_boy]

Low current cut-off circuit

I also need some advice on using comparators:

I'm trying to design a simple circuit that cuts-off when the current through a time-varying load falls below a threshold value.

Basically, I am electrochemically etching a piece of wire, which initially has a resistance of ~1k Ohms. When the etching is complete the resistance rapidly (few 100ms) increases to essentially an open circuit, at which point I want to automatically break the circuit. The etching requires a constant voltage of ~3V dc.

I've tried to design a comparator circuit to do this, but the problem is that the input (sense) voltage has to have some feedback from the output circuit driving the etching, and my mind starts melting.

I can attach rough schematics if anybody feels they can help.

Cheers.

 

[pi_boy]

I've attached a current-time plot of the current through a wire at the moment - there is a sharp drop to zero when the etching is complete. I want to be able to automatically cut-off the voltage at this point.

Also a rough schematic - apologies for any glaring errors, my grasp of electronics is just about plug wiring level. The pot on the left is used to set the cut-off threshold, and the one on the right simulates the wire being etched.
I'm not sure if I have the 'Vin' connected properly - this needs to sense the changing voltage (current) through the test resistor.

I've just chosen an arbitrary comparator as I don't really know the difference between them.

Any comments would be greatly appreciated!

 

[philba]

google is your friend. 3rd hit - **broken link removed**

pi_boy, I suppose you jumped on this thread because you can't start your own? what you want is simple. I think you should seperate the sensing and the etching. use the sense wire as the top half of a voltage divider driven by your supply voltage. I'd try a 10K ohm resistor for bottom half. the voltage at the middle will start out at Vcc (your supply voltage) and drop to 0. that is one comparator input. the other comparator input should be Vcc/2 from a 10K/10K divider (or make it a pot to adjust the cut-off point). The comparator output then drives a switch for your etch voltage. You could use a relay or transistor for the switch. Look at the single supply diagram in the above link. I'd build in hysteresis to prevent noise from causing the voltage to "stutter". the above link explains this in more detail.

edit: you didn't have the diagram up when I first posted. do you have the grounds from the two sides connected? also, I would definitely add hystersis (it's just a resistor). On the left-hand side, you are showing a 10K pot and 100 ohm resistor, is this in addition to the wire being etched? Your 1K pull up on the comparator output seems kind of low. the 10K base resistor will limit your transistor's collector curent to something on the order of 10mA. Is this what you wanted?