Basically I'm looking for a cheaper solution to the UDN2981A, which allows a 5V input to drive a load upto 35V. It acts like a buffer, but with a level shift rather than an NPN transistor arrangement in the ULN2003A. The loads I'm driving (magnets for a pipe organ chest) are all common ground, and the easy solution - making the system common +ve is not possible due to the protection diode on every magnet. Each magnet draws a maximum of around 250mA. The organ, like any other runs at around 15V, but the logic system can only run at 5V. I've had a play with various transistor arrangements, but all seem a little long winded. Any suggestions? 5V input = 15V output, 0V input = 0V out or open circuit. The less additional transistors the better. Thanks in advance.
SOLUTION 1: Using a PNP to drive the coil. This works, but I'd prefer to have an NPN to drive the coil as I have 100's of NPN's. Any suggestions?
Can't think of a more economic solution - but don't you need a base resistor on that PNP? edit: Tried an NPN emitter-follower for the second transistor - with a fairly high-gain Q2 (2N2222A) and a low-value load (150R) on Q1 (2N3904) for a low voltage drop when Q1 is off, almost 14V got to the relay. You'll need to invert the logic, low input puts relay on.
Yeah I missed the base resistor off on the schematic. Inverted outputs from the driver circuitry were what I'm trying to avoid as it could leave every note on the organ playing at once while the system is initialising or if there was an error!! Not great for the PSU supplying the magnets or people's ears!! Thanks anyway. Time to start making 1000 of these things!
