Let's Make Robots!

Simple and sensitive finger-type pulse detector

Measures your pulse/heart rate

The circuit shows a sensitive finger-type pulse detector. The concept of the circuit is simple. With the alteration of the blood volume in the finger the contribution of the incident light on the LDR also changes. The alternation of the resistance causes an alternation of the voltage by a voltage divider (R1/LDR). The small alternation of the voltage will be then highly amplified by applicable op-amp stages.

I have chosen the op-amp-IC LM 358, because this IC includes already two op-amps, it works in a wide operating voltage range and this op-amp type does not need a negative operating voltage. Both op-amps are wired as non-reverse amplifier. The amplification factor of IC1A is 100 and the amplification factor of IC1B is approximately 560. The amplification can be adjusted by the potentiometer R6. The output signal on pin 7 of the second op-amp triggers the transistors T1.

You may have already recognized, that there is no bright LED in the circuit included to screen your finger. Normal ambient light during the day or bright room lighting at evening is enough! This makes this circuit perfect for robot use.

One idea would be to 'program' your robot with your heart rate. The procedure could be as follows:

a) Robot is waiting to measure your heart rate

b) Put your finger on the sensor --- warning beep --- measuerment starts

c) Second warning beep --- measurement finished

d) Robot reacts acording to your heart rate. For instance: heart rate < 60 bpm --- robot reacts and moves slow; heart rate 60 to 90 bpm --- robot reacts and moves medium; heart rate > 90 bpm --- robot reacts and moves fast

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Hi,

I need this circuit for my project, but I need some explanations of the circuit if you please.

1) are the amplifiers used only to amplify or to amplify and work as band-pass filters?

2) also how is the gain of the first amplifier calculated?

3) in case the op-amps are used as a band-pass, then how is the high and low cutt-off frequency calculated?

4) what is the 1 micro farad capacitors used for?

Regards

Hi,

I need this circuit for my project, but I need some explanations of the circuit if you please.

1) are the amplifiers used only to amplify or to amplify and work as band-pass filters?

2) also how is the gain of the first amplifier calculated?

3) in case the op-amps are used as a band-pass, then how is the high and low cutt-off frequency calculated?

4) what is the 1 micro farad capacitors used for?

Regards

I saw you are using the BC547.  However, is this the BC547B or BC547C version?  There is a very big difference in the amount of amplification.

 

Regards,

Xander

I am using a BC547C, but in this case you can also use a BC547B as no high gain is required.

Updated the circuit, named the npn transistor "T1" instead of "Q2".