Andrew Cowan
Senior Member
I'm using an 18X (running off 5V) to control two cascaded 74HC595 shift registers (also running of 5v).
These then feed into two ULN2803 darlington drivers, which are connected to 16 LEDs. I'm running the ULN2803 drivers off 3.1v, so I don't have to use limiting resistors on all the LEDs.
However, there seems to be a problem - the brightness of the LEDs. The positive side of the LEDs are getting the full 3.1v, which shows the problem is with the darlington's negative switching.
Looking at the voltage of the LEDs with a scope, the negative line seems to be around 1v when the LED should be on - not 0v as it should be.
I wired the circuit up quickly at school today and didn't have long to troubleshoot. I'm sure the problem is something small I've done wrong, but I can't get back to the circuit before tomorrow.
Does anyone have any suggestions in the mean time? The PICAXE/shift register stage is fine and runs direct from 5v.
Andrew
Edit - just thought I'd add that I've measured 0V directly on the ground leg of the ULN2803.
These then feed into two ULN2803 darlington drivers, which are connected to 16 LEDs. I'm running the ULN2803 drivers off 3.1v, so I don't have to use limiting resistors on all the LEDs.
However, there seems to be a problem - the brightness of the LEDs. The positive side of the LEDs are getting the full 3.1v, which shows the problem is with the darlington's negative switching.
Looking at the voltage of the LEDs with a scope, the negative line seems to be around 1v when the LED should be on - not 0v as it should be.
I wired the circuit up quickly at school today and didn't have long to troubleshoot. I'm sure the problem is something small I've done wrong, but I can't get back to the circuit before tomorrow.
Does anyone have any suggestions in the mean time? The PICAXE/shift register stage is fine and runs direct from 5v.
Andrew
Edit - just thought I'd add that I've measured 0V directly on the ground leg of the ULN2803.
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