Design of Square pulse generator using LM358N and LDR

[aswathymohan]

Hi

I have seen a circuit diagram used for heart beat monitoring system using Ldr and lm358n.Can any one tell me about the design of resistor and capacitors used there.?My objective is to count the number of pulses coming from the outpin pin (7) of lm358n.

 

[BradtheRad]

The resistors need to have an ohm value that is in approximately the same range with the photocell resistance.

If their value was small, it would involve large current flows, making it impossible to detect changes in the small currents flowing through the photocell.

The capacitor C2 is commonly advised for the purpose of reducing noise on the supply line (either incoming noise or noise caused by the device itself).

 

[aswathymohan]

Thanks for ur response..What is the purpose of capacitor C1??

 

[BradtheRad]

Sorry, I didn't see a capacitor before but now I can see where it has a label 'C1' in your diagram.

Its value appears to be 470nF.

I believe it is a DC blocking capacitor. It allows the op amp to be very sensitive to tiny variations on the incoming signal.

The value apparently is chosen to allow low frequency AC (heartbeat signal) to pass to the op amp.

The sensitivity adjust is important, in order to make the output signal swing up and down a few volts.

 

[aswathymohan]

How the square pulses are coming from the 7th pin when switch is closed??

 

[BradtheRad]

How the square pulses are coming from the 7th pin when switch is closed??

The second op amp is set for maximum gain (because there is no feedback to the inverting input).

If a small signal (or large signal) comes from the first op amp, the second op amp will amplify it further so that its amplitude is the same as the supply voltage.

 

[aswathymohan]

Is the 2nd opamp acts as a comparator??

 

[BradtheRad]

Is the 2nd opamp acts as a comparator??

Yes, you could call that comparator mode.

However since they are op amps, you'll get the most usable response by powering them from a split supply.