I'm building a simple metal detector. There is a Colpitts oscillator with a confirmed (measured on CRO and simulated) frequency of 62 kHz. I now need to take this base frequency and detect changes, which indicates the presence of a metal nearby. When a metal is near, the inductance increases and the frequency drops.

I've used a voltage follower to buffer the output of the oscillator. Then fed this buffered output into an infinite gain multiple feedback active bandpass filter using a TL074CN op-amp. Datasheet Link

The intent was, the base frequency would be output by the op-amp which would switch on a BJT that would short-circuit an LED. If the frequency changes, the op-amp output would drop, switch off the BJT, and the LED will switch on as the current into the collector will go through the LED into the ground.

The bandpass filter designed values are centre frequency 62 kHz with a bandwidth of approx 7 kHz, quality factor of 9, and gain is 170.

Neither the LTSpice simulation nor the built model seems to work. The simulation output is flat at 1.5 V.

The breadboard model doesn't work either, it seems the voltage output of the filter is too high it consistently switches on the BJT. Removing the base wire confirms that the BJT and LED are all working. I've tried with a 3 V and a 9 V supply.



I've redesigned it to take a 9V battery, so +/-4.5 V rails. To do this, I used two 68k Ohm resistors for a voltage divider, with a wire connecting to the ground rail connected to the node in between them. However I am now not getting any power delivered to the components. The battery and all other components (LEDs, op-amps, BJTs) work when the negative terminal is connected to the ground. I have an ON indicator LED that simply switches on when there is a power supply which should not be affected by the rest of the circuit yet it still is not switching on.

Voltage Divider

The voltage divider resistors and wire are encircled.

  • \$\begingroup\$ Maybe the R8 30k resistor should be connected to ground? \$\endgroup\$
    – Willis Lin
    Commented Apr 19, 2023 at 1:16
  • \$\begingroup\$ The oscillator itself works perfectly, it's just a question of processing the output from the inductor. \$\endgroup\$
    – Ale
    Commented Apr 19, 2023 at 1:18
  • \$\begingroup\$ Are you trying to make a voltage follower? If yes, you should connect the input to positive input and feedback to negative input. \$\endgroup\$
    – Willis Lin
    Commented Apr 19, 2023 at 1:32
  • \$\begingroup\$ Confirmed, the red wire from the green capacitor is the output of the inductor. It feeds into pin 5 which is one of the non-inverting inputs. There is a small wire hidden beneath the blue one connecting pins 6 and 7 together, the inverting input, and output of that op amp. The blue wire in between them is the voltage follower output that inputs into the two 100 Ohm resistors in series. \$\endgroup\$
    – Ale
    Commented Apr 19, 2023 at 1:42
  • \$\begingroup\$ Seems like TL074 output low voltage level is higher than 1V. This voltage level is not low enough to turn off a NPN. You can use voltage divider to lower the NPN base voltage. \$\endgroup\$
    – Willis Lin
    Commented Apr 19, 2023 at 2:17

1 Answer 1


Need a voltage divider to the base or 2 series forward biased diodes in series with the emitter. The TL074 swing is to within 1.5V of supply. https://www.ti.com/product/TL074


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