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Need help for a heartbeat detector!


Fate

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:-[Hmmm... Now I want to amplify it (a lot).


Hi Fate,
Look at "B" opamp, no negative feedback. At the low frequency of heartbeats, its gain must be 100,000 or more. You won't have to amplify it.

... feed the output to a comparator

Look again at "B" opamp. It is a comparator, and it even has an adjustable threshold!
It probably already has "proper pulses" so just feed them to a counter.

This circuit is self-contained and has everything that you need. That's why I selected it for you. ;D
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Hi Bird,
The output type of an LM358 opamp is a complementary push-pull emitter-follower stage like most other opamps. See its schematic attached. The LM358 and its big sister, the LM324 quad opamp, is unique in that it can operate with a single supply voltage as low as 3V, and has a very low idle current. In this circuit it operates at 5V.

Opamp A operates as an ordinary AC non-inverting amplifier, with its gain adjusted by the sensitivity pot.
Opamp B operates as a comparator, with an adjustable trigger level.

I guess the sensitivity and trigger level pots allow the circuit to match the enormous difference between different fingers. Probably when a finger is cold, it has low blood-flow.

For its display, you need a counter circuit with a timebase measuring pulses per minute.

post-1706-14279142142765_thumb.gif

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Hey Trigger...
The optical technique presented here exploits the fact that tiny subcutaneous blood vessels (capillaries) in any patch of skin (fingertip, ear lobe, etc.) furnished with a good blood supply, alternately expand and contract in time with the heartbeat. An ordinary infrared LED/phototransistor pair can sense this rhythmic change as small but detectable variations in skin contrast. That's pretty much the gist of the theory it's based on...

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Hi Fate,
TTL counter ICs require the voltage from a logic low source to be 0.8V or less. The output of a 741 opamp won't go that low, probably 1.0V or more. The other opamps have outputs that go down to within millivolts from ground.

I lost the whole article but the PIC was probably used as a pulse counter with a 1 minute timebase.

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drugstore?? dun understand... i constructed d circuit..anyway i cant get wat it suppose to be.. i test d heartbeat using oscilloscope.. i oso dunno how many value should i put on both potentiometer

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