Question About 555 Timer Frequency Meter Circuit

[eev_carl]

Hi,

I was wondering if someone could help me troubleshoot this circuit.

I breadboarded the attached frequency circuit, but the output I'm getting differs from the schematic's description.  I expected to see a linear rise in output current with an increase in frequency, but when I increase the frequency, I see the current fall in steps to 0uA.  I've tried a few ranges because the description mentioned non-linear behavior at a certain point.  One such range is 1kHz - 5kHz by 1kHz steps.  I also tried increments of 100Hz.

My current circuit has C3 = 0.1uF and R3 as 10k.  My R5 is set somewhere around 4k.  My input source is a 4Vpp pulse offset by +2V.  I'm measuring the current with a multimeter.

Thanks,
Carl

[StillTrying]

I think that will only work as a frequency meter if the input signal is narrow (<100us) pulses.

[Zero999]

Are you using a digital meter?

An analogue meter must be used to measure the current.

[eblc1388]

The 555 timer acts a monostable with a fixed output high duration. This timing is controlled by R3/C3 values. Care must be taken to ensure that this "high" timing does not extend longer than the timing of the input trigger period or else the 555 timer cannot reset between triggers.

I changed the C3 value down to 10nF to give a shorter timing and did a simulation. The result is as expected. Meter current changes from 100uA to 900uA for a change of input frequency from 1KHz to 9KHz in steps of 1KHz. THe odd looking value of R5 is to "calibrate" the meter to read exactly 900uA at 9KHz.


 

[eev_carl]

Are you using a digital meter?

An analogue meter must be used to measure the current.

Yes, my multimeter is digital.  I don't have an analogue meter on hand.  Wouldn't I see some current fluctuations on the digital multimeter because of the ongoing sampling?  For my current circuit, going from 1kHz-2-3-4 displays .22mA-0-.07-0. I don't see any changes when I pause between frequency settings.

[eev_carl]

The 555 timer acts a monostable with a fixed output high duration. This timing is controlled by R3/C3 values. Care must be taken to ensure that this "high" timing does not extend longer than the timing of the input trigger period or else the 555 timer cannot reset between triggers.

I changed the C3 value down to 10nF to give a shorter timing and did a simulation. The result is as expected. Meter current changes from 100uA to 900uA for a change of input frequency from 1KHz to 9KHz in steps of 1KHz. THe odd looking value of R5 is to "calibrate" the meter to read exactly 900uA at 9KHz.

Thanks for simulating this for me.  I'm still not getting the output current to track with the input frequency even with your values and calibrating the pot.  I'm going to set this aside and borrow an analog multimeter as suggested in a different reply to see if the current is changing too much for my multimeter.

[StillTrying]

Yes, my multimeter is digital.  I don't have an analogue meter on hand.

As you can see on eblc1388's circuit he's added a 100uF across the meter connections to smooth the current reading, try that with your DVM, you might have to use an even larger value.

I changed the C3 value down to 10nF to give a shorter timing and did a simulation. The result is as expected. Meter current changes from 100uA to 900uA for a change of input frequency from 1KHz to 9KHz in steps of 1KHz. THe odd looking value of R5 is to "calibrate" the meter to read exactly 900uA at 9KHz.

I found out the hard way that any calibration is very V+ dependent. The +5 to +15 would have to be regulated.


Input stage added with only 280mV hysteresis, so that it works with any input wave shape from 0.3Vpp to 20Vpp - in theory.

[eev_carl]

As you can see on eblc1388's circuit he's added a 100uF across the meter connections to smooth the current reading, try that with your DVM, you might have to use an even larger value.

I have the 100uF capacitor.  Is there anything I can check on a digital scope to troubleshoot? 

[StillTrying]

Is there anything I can check on a digital scope to troubleshoot?

Of course, if you apply a 1 or 2kHz signal to the input, and then look at the output pulses on pin3 of the 555, you should see ~90us wide pulses something like that.
As you vary the frequency of the input the pulses should move closer or further apart, but their width should stay the same ~90us.

[eblc1388]

For my current circuit, going from 1kHz-2-3-4 displays .22mA-0-.07-0.

It appears that the 555 is not triggered properly at higher frequency. What is the supply voltage for the 555 timer?

If it is at +12V, then the idle voltage is sitting at +6V. The input signal must cause it to drop below +4V in order to properly trigger the circuit. If the pulse falling edge is not sharp, then it might not been able to result in a proper trigger.

Try reducing the 555 timer supply voltage from 12V to 6V, the trigger point reduces to +3V and a 1V drop will trigger it. Or else you have to add some input conditioning before feeding the signal to the 555, as pointed out by StillTrying.

[Jwillis]

They usually run between 4.5 volts and 16 -18 volts depending on the prefix on the chip.But 6 to 12 volts is common.