Project Board outputs

[BruceT]

Hi, I am hoping that this question is not too stupid!!! I have looked through the manuals.....
On the project boards (the 18 for example) the outputs from the darlington chip are negative (0 volts) and then there is a common + rail. My problem is that I have a camera with three connections - a common ground (0 Volts) and the other two 'trigger' lines ( 1 to activate and 2 fire the camera) are positive. So I can't connect the two trigger lines to the darlington chip outputs. I also need to keep the two lines separate and so it's of no use connecting them both to the positive rail.

The more I look at it, the more I wonder! Is this not a very common problem with the way these outputs are designed???

Sorry, if I am not seeing the obvious answer!!!

Many thanks, Bruce

 

[AllyCat]

Hi Bruce,

The Darlington ic is intended as a "driver" for high(er) current devices like motors or power LEDs, etc., so you may need to remove it (from its socket). But note that the "+ve rail pin" is misleading, it only connects to the output "protection" diodes. The outputs are intended to use / require "pull-up" resistors, or to "sink" current from an extenal load (connected to a higher voltage).

Therefore you might still be able to use the Darlington Outputs to drive your camera, but more normally it would be driven "directly" from one or more of the PICaxe pins (which should be brought out to a header/connector, or maybe from the empty Darlington socket). However, my term "directly" might mean via a suitable resistor, or some other component(s) such as a transistor, FET , "bilateral switch" (ic) or relay, etc..

Cheers, Alan.

 

[BruceT]

Hi Alan, thanks for that. I was wondering if I should use a transistor to do the switching, but just wanted to make sure that there was no way to use darlington directly. I didn't want to needlessly duplicate functions. Thanks again, Bruce

 

[inglewoodpete]

An alternative "High-side" Darlington driver could be used. If you understand what you're doing, you could use a UDN2981.

BUT you would need to splay out legs 9 and 10 on the UDN2981 chip so that they do not get inserted into the CHI030's socket (intended for a ULN2803). Then connect pin9 (Vs) to the positive camera supply and pin10 to 0v using short lengths of hookup wire. Note that these two connections for the UDN2981 are the reverse of the ULN2803, since the UDN2981 is a high-side (+ve) Darlington driver!

Do so at your own risk, though! Read and understand the UDN2981 data sheet first.

 

[BruceT]

No worries! Thanks. I have been experimenting with transistors and still have some weirdness happening so my next step will be optocouplers. I can't see how they could have any problems! Many thanks, Bruce