im having a bit of trouble with the following circuit. i am using an LED to shine light onto our finger and reflect onto an LDR (RD). the LDR is the source of the signal. the voltage input is in the 20mV range and the amplification should be 100 times to get it up to 2V. however we are getting no amplification and filtering is also not working. we are currently using a high pass filter followed by a 2 stage low pass filter.

LDR characteristics: - Dark resistance: 0.5M ohm min. - Light resistance: 2.8K ohm min 8.4K ohm max

the setup

circuit diagram so far

oscilloscope reading

Power supply setup

  • \$\begingroup\$ Are you trying to use +- appx 6V dual supplies? Don't you need to connect "+" from the negative supply to "-" of the positive supply for your DC common? \$\endgroup\$
    – Tut
    May 21, 2014 at 11:15
  • \$\begingroup\$ oh ok, will try that! was having similar issues when using a single sided supply also. \$\endgroup\$ May 21, 2014 at 11:55
  • \$\begingroup\$ Also, in case that isn't the only problem can you show the full circuit diagram with power rails to the op-amps and decoupling caps on the op-amp power rails. No short-changing! \$\endgroup\$
    – Andy aka
    May 21, 2014 at 13:50
  • \$\begingroup\$ Also, from the picture of your setup, it looks as though you are using electrolytic caps for your filters. These do not work well with AC. Replace them with non-polarized caps (ceramic ought to work OK for this application.) \$\endgroup\$ May 21, 2014 at 21:34
  • \$\begingroup\$ Very general advice - follow the signal through each stage and see where things go wrong. \$\endgroup\$
    – user32885
    Jul 28, 2014 at 8:27

1 Answer 1


The problem is the high pass filter after the first amplifier. With a 1uF capacitor and a 3.2K resistor the cutoff frequency is about 312 Hz. Your frequency input is 0.5 Hz to 30Hz and is being attenuated by the filter. At 30 Hz, the gain of the high pass filter is 0.1, so for 20mv in you get 2 mv out. at 3 hz the gain is 0.01 and at 0.3 Hz the gain is 0.001.

For your circuit, the high pass corner frequency will have to be no more then 0.1 the frequency out from the sensor


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