Help with Picaxe020 project board

C

cdngunner

Senior Member
Hi folks

Have not been on here in a while and trying to get my Micro-controller brain restarted.

I am building a brass annealing machine. I am using the Picaxe020 to control the stepper motor in the project. I am using 12VDC secondary voltage for the L293D and the ULN2803A. Yes both chips are in at the same time.

I am trying to use the left over outputs via the ULN2803A. (b0-b3). The output goes hi but I only see 0.5 volts. The outputs, b0,b1, go to a voltage divider which then drives a TIP122. Voltage divider drops 12VDC to 5VDC. TIP122 circuit works, it has been tested by itself.

EDIT: the TIP122s drive 24vdc solenoid valves

So any idea why I am only getting 0.5 volts???

Thanks guys ( I know that I am missing something, just can't put my finger on it)
 
Last edited:
C

cdngunner

Senior Member
OH MOTHER OF PEARL !!

How the heck did I miss that. The ULN is a Darlington and will not work the way I want it to. Anyone have a work around


I guess I could tear out the ULN and jumper the header so that I can use 5vdc the outputs directly. And get rid of the voltage dividers
 
W

westaust55

Moderator
You could try adding a resistor from the supply (5Vdc or 12 VDc) to the ULN2803 outputs as a pull up resistor.
But then the signal at the ULN2803 are inverted so that setting. PICAXE output high results in a low on the ULN2803.
A potential problem is on power up until the PICAXE chip has initialized all your solenoids would momentarily be ON.

There is a high side electrical equivalent to the ULN2803 but it is not a pin for pin identical pinout so your boards will need some mods.
 
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