Hi I'm new to these forums. I'm no electronics expert but I do understand technology to a good level

I want to use an LM2907 as the first stage in a larger circuit.
So I've used the circuit here on a breadboard
https://i0.wp.com/microcontrollerslab.com/wp-content/uploads/2015/09/Configuration-for-LM2907.jpg

I want to bench test this, so I've rigged an Arduino microcontroller with the code found here https://www.arduino.cc/en/Tutorial/BlinkWithoutDelay
to generate pulses

The pulse generator side of this works fine - I can see them on an oscilloscope, but I get nothing on the Voltage output on the LM2907 circuit.

I've had 3 goes at putting this together now with two separate LM2907 chips (in case I'd blown one), but with the same result.

Actually the voltage output is not quite zero. My Digital multimeter reads about 3mV, but that figure is constant no matter what frequency the input is set to.

The only problems I can foresee as the possible reason for this are that the impedance of the input is wrong or that the 5v output voltage is not high enough from the arduino.

Could that be the case and what could be done about it?

Any help very much appreciated!

 

The circuit shown uses a magnetic transducer, not a TTL input - how have you connected the input signal?

The chip expects the input to go below ground, your input signal is not triggering the chip.

Depending on which variant of the part you are using, you could get it working by:

1) capacitively coupling the signal to the input

2) biasing the negative comparator input above ground with a resistor divider

read the data sheet!

 

Thanks for that reply Sensacell. As I mentioned I'm no electronics expert so the data sheet isn't much help to me - I need advice (please).

Of course the application in the diagram is for a rev counter which is why it has a magnetic pickup, but since presumably these LM2907 are multipurpose, I was assuming that wasn't an actual requirement. Often Tacho pickups are connected to the switched side of the ignition coil.

If the small (in comparison to the 12v on the LM2907) 5v square wave from the Arduino is not the problem, I imagine it may be that the LM2907 expects to 'see' a switched short circuit to earth and that the Arduino passes no significant current and effectively has its output 'held' up by the LM2907. I can see your Option 1 prevents that.
Your option 2 I understand, but what resistor values would you suggest? I assume that may depend on the current needed to make the LM2907 react.

I had wondered if a transistor might be required to amplify the Arduino output - but I have no idea what components would be needed

Any further help gratefully receieved

spegru

 

If you have the 8 pin version of the chip you should use option 1 from post #2
If you have the 14 pin version you should use option 2 from that post.

Which version do you have, then we can get down to cases?

 

Thankyou AlbertHall. I didn't think that the package would make any difference
But in fact I have both types, although the 8pin package easier for me to follow since that is what's used in the circuit I have/linked to
Can you suggest component values please?

 

Connect your arduino output pin to pin 1 of the '2907 with a 10uF capacitor with the positive of the capacitor connected to the arduino. Then connect a 10k resistor from pin 1 of the '2907 to ground (pin 8 of the '2907).

 

Brilliant I'll give that a try!
Thanks

 

Thanks very much indeed AlbertHall. That worked perfectly!
I can now alter the pulse frequency using the Arduino IDE and alster the voltage output at the LM2907
If I could just ask for a bit of explanation:
I understand capacitive coupling but I don't understand why it was necessary?
The 10k resistor is not a pull-up since it's going to ground, so what is that actually doing?

Thanks again

 

The arduino produces a signal which goes from 0V to 5V. Consider if the capacitor and resistor were swapped so the resistor feeds the capacitor forming a low pass filter. The capacitor would end up with the average of the arduino signal, which would be 2.5V if the signal is a 50:50 square wave. The same happens when they are the other way round with the capacitor having 2.5V across it. Now when the arduino signal is at 0V the other end of the capacitor is at -2.5V and when the arduino signal is at 5V, the other end of the capacitor is at +2.5V. Thus the input to the '2907 is bipolar going above and below 0V as it requires.