Drive Analog Panel Meter with PWM from Panda II

I’m seeking some design assistance with a project involving a Panda II and an analog panel meter. In fact, it is this meter:

My goal is to be able to drive the position of the meter using the PWM output. The accuracy is not critical, but (ideally) a PWM duty cycle of 0% should result in the meter being at it’s resting position (or very close) and a PWM duty cycle of 100% should be at the full-scale deflection position (or very close).

Since the PWM output is 3.3v and full-scale deflection on the meter is at 5v, I need to bump up the voltage. For that, my plan is to switch the 5v rail coming off the Panda II board to the meters using a NPN transistor (like a 2N2222) in the way you would expect (PWM output to base through a small resistor of 470 ohms, emitter to ground, meter between 5v rail and collector).

The meter reportedly draws 0.75mA at 5v.

Some questions about this plan:

  1. I assume that varying the duty cycle between 0% and 100% will linearly position the meter. That is, at 10%, I would expect the voltage to be 0.5v, at 20% the voltage will be 1.0v, etc. Is this correct?

  2. I assume that if the PWM frequency is around, say, 500Hz, the physics of the meter will effectively smooth the signal-- I don’t need to add any capacitors to keep the meter from visibly wigging. Is this correct?

  3. I assume that because the meter is effectively a coil around a magnet, that it presents an inductive load to the transistor. It’s probably very small, but because of this, I imagine I need to protect the transistor with a small diode. Is this correct?

I’m an experienced software engineer and comfortable with digital electronics. I’m less comfortable with the analog side of things and would appreciate any insights.

Why not use the analog output pin?

idd, use analog output pin with an OPAMP and trimmer potentiometer to adjust the gain.

If you don’t have analog out or you want to use a PWM, I would also connect the above circuit, but first create a DC voltage from the PWM with a series resistor and a parallel cap. Then feed that DC voltage to the OPAMP.

When you select an OPAMP, make sure it can set it’s output very close the V+ rail as you will probably want to feed it from a 5V source.

I am driving six meters from the six PWM outputs. I am using the analog output for another purpose.

I appreciate offering other solutions, but I was specific for a reason.

Thanks for the suggestion of using an op-amp. If I cared about accuracy, I might consider it, but given the inherent inaccuracy of these meters, I would think a simple transistor as I described would be a more minimal solution.

With a transistor you loose 0,6 V of your full scale. I would use an n- channel fet instead. And put a reversed diode in parallel with the coil of the meter.

Sounds like a cool project!

I’d use a ULN2803 as the buffer. It has the protection diodes built in and is cheap. At 500mA total it should handle 8 meters at 40mA each easily.

I think the darlingtons have a lower voltage drop too so you should get your full scale deflection. If not, drive from 6v or 12v and scale your pwm in software.