Is it possible to make the arduino analogue channel read -1v when no voltage is applied ?
I want to build a signal generator but the device i want to trigger needs a zero point crossing so when the volts drop to 0v it is not enough to trigger a pulse
Do i need to use a transistor and wire it to 5v and -5v
Is it possible to make the arduino analogue channel read -1v when no voltage is applied ?
No.
The input will only measure positive voltages.
Do i need to use a transistor and wire it to 5v and -5v
Well an op amp or two would be better because then you can control the offset voltage, but you must always ensure that the voltage you feed into the input is no lower than zero.
Can you define the voltage levels and trigger requirements more clearly?
For bipolar voltage inputs, a comparator and bipolar power supply is usually required.
I should have been more specific
I am using a analogue pwm channel to output a voltage (not to read a voltage)
I would like the channel to drop below zero volts when it is not outputting
I am using it to produce frequency pulses
The device reading the pulses needs a zero point crossing
Use an op amp on the output to shift the waveform to go from say -1 to +5v .
Yes you need an op amp fed with a split supply. Then put a negitave DC offset on it so that when you output 0V you get -1V out of it.
This might help you:-
Thank you for the advice guys
Gadgeroonie:
I am using a analogue pwm channel to output a voltage (not to read a voltage)
I hope you understand that PWM is NOT a voltage, but a digital signal.
Leo..
If you input trigger is relatively high impedance and it only needs a voltage no more that 1/2 volt below zero it can be done with resistors.
Warning, I've not tried this method but theoretically it should work.
Approach:
Using a 5K to 10k resistor, pull the output pin voltage to -5V ! Wow you say, is this guy crazy! (maybe).
What will happen is the output when off will be limited to a negative 1/2 volt by the internal diodes in the processor. When you energize the output it will go to ~ 5V.
As for the output being PWM and not analog as @WAWA mentioned that needs to be addressed based on what your trigger is really expecting.
Question. If you are trying to "trigger" your signal generator do you really need an analog output? Could you not just use a digital output. The same -5V trick will still work.
It's easy to make a low-current negative 4.5volt supply with another PWM pin and a few passive parts.
Leo..
