Hi there,

I'm using an Atmel AT89S8253 to drive a 24VDC solenoid valve via a TIP120 darlington pair, but am having a few problems with the output of the AT89S8253. I have 2 circuits on seperate boards that are as follows:

The first circuit contains the AT89S8253 and its associated hardware. I am using P3.0 to drive the TIP120. My program appears to work correctly when I measure the voltage on P3.0 without any load (so nothing connected to P3.0), i.e. P3.0 goes to +5V when instructed to, and goes to 0V when instructed to.

The second circuit contains the TIP120; a 10k resistor between the input (from P3.0) and the base; and a diode across the solenoid connections. This circuit has been tested using its 24V supply and a seperate 5V supply, and is working correctly.

So far so good, then - everything appears to be working as it should. The problem comes when I connect P3.0 to the input of my TIP120 circuit. When it is connected, and I instruct P3.0 to go high (which should switch my solenoid on), the voltage on P3.0 only goes to about 2.2V and my solenoid valve does NOT switch on. P3.0 is still turning on and off as instructed, but only reaches 2.2V in the high state. The 5V supply (for the AT89S8253) and the 24V supply (for the solenoid) share a common ground.

I fail to see why the voltage is not reaching the full 5V and switching my TIP120 circuit, when as stated previously, P3.0 appears to be working correctly without any connections. If anyone is able to provide a little insight into what might be going on here, it would be much appreciated. I hope everything is clear, but if not, I'll try to provide more detail.

Thanks
Warren