Complete newbie needs help with LM3915 VU Meter Circuit

audioguru said:

I never buy cheap Chinese fakes or counterfeits from ebay, Amazon or Ali.

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Went to a electronics store and bought some proper 1N4148s ESD packaged and everything. Still only get 0.2V drop. Checked the datasheet and voltage drop is around 0.6V for 0.1 milliamps.
Tested the diodes outside the circuit with 0.1 milliamps and got the intended 0.6V drop for all diodes, including the repackaged China ones. I then measured the current flow through the diode in the circuit and only got 6 microamps.
Is it supposed to be that low? Or did I screw up my wiring somewhere?

Muoshy said:

I then measured the current flow through the diode in the circuit and only got 6 microamps.
Is it supposed to be that low?

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Seems reasonable, since Vcc is 9V, R7 is 1MΩ and the input impedance of pin 5 is very high.

National Semi says in their datasheet for the LM3915 that the very simple transistor and diode peak detector produces an offset voltage of only 0.1V. But I simulated it and got hundreds of mV. Then a few LEDs in your circuit will light all the time. Now I call it Mickey Mouse peak detector circuit.

I never used that Mickey Mouse circuit, I always use a precision peak detector circuit with a single supply opamp and a transistor. The transistor can charge the peaking capacitor much quicker than the 10k resistor in the Mickey Mouse circuit and it has an output offset voltage of only a few mV. Here is my peak detector circuit that works perfectly:

Thanks! Any equivalents to the MC33171? Not sure I can get one.

Muoshy said:

Thanks! Any equivalents to the MC33171? Not sure I can get one.

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There are many modern rail-to-rail opamps but most are surface-mount without pins. One of the lousy old opamps in an LM358 dual opamp will be fine for the VU meter since it does not produce the crossover distortion and its poor high frequency response does not matter. Disable the unused second opamp by connecting its (+) input to 0V and connect its (-) input to its output.