ULN2803 inputs

[john thorne]

Can I connect the outputs from 3 picaxe 18m2 to one ULN2803 input.
Effectively giving me an or gate.
Thanks

 

[hippy]

Welcome to the PICAXE Forum

You cannot connect outputs directly ( or rather shouldn't ), but you can put a diode in each output line to create a diode-OR gate. Anodes to PICAXE, cathodes joined, to ULN.

 

[john thorne]

Thanks

Welcome to the PICAXE Forum

You cannot connect outputs directly ( or rather shouldn't ), but you can put a diode in each output line to create a diode-OR gate. Anodes to PICAXE, cathodes joined, to ULN.

Thanks for the quick reply. That will work for me.

 

[inglewoodpete]

If you have outputs to spare in the ULN2803, then you don't have to use diodes. Use the open collector darlington transistors in the ULN2803.

Connect 3 PICAXE outputs to 3 inputs on the ULN2803. Then common the 3 associated darlington collectors together. The load, of course, will go between the +ve rail and the collectors. Turning on any darlington will energise the load.

Finally, there is another alternative: do the "or" function in software and use a spare PICAXE output to drive a single darlington.

 

[Buzby]

Finally, there is another alternative: do the "or" function in software and use a spare PICAXE output to drive a single darlington.

An example of two different interpretations of the OPs original text - "outputs from 3 picaxe 18m2".

My interpretation was 3 seperate 18M2 chips, not 3 pins on one chip.

Obviously the software 'or' function will only work for 3 pins on one chip.

The diode option, or the 'commoned collector' option, will work on both single and multiple PICAXE chips.

 

[Technical]

Connect 3 PICAXE outputs to 3 inputs on the ULN2803. Then common the 3 associated darlington collectors together. The load, of course, will go between the +ve rail and the collectors. Turning on any darlington will energise the load.

This is the way we would also recommend. Remember to link all 0V rails together.