Friday, 6 July 2012

Pull down

Just as a quick note to myself: the UDN2981 requires pull down resistors as the inputs are not allowed to float when Vcc is powered. A SIP resistor pack should sort that without taking up too much space.

Tuesday, 3 July 2012

Relays

It turns out you can get relays from Farnell for about 55p each. You need two SPST relays per point, plus something that can switch the 30mA coil current @ 12V DC. Having played about with PCB schematics and vero-board layouts, I'm starting to think relays would be a more robust option than the L293 drivers. It also frees me up to purchase standard Peco motors rather than the 2A 'E' high-efficiency motors, and reduces concerns with regard to supplying the motors from a capacitor discharge unit. I may even be able to throw two points in a crossover on one output. Whereas the signals are common cathode, the relays are single coils which work in either polarity. As the UDN2981 (8 way high-side switch) and ULN2803 (8 way low-side Darlington array) are largely pin compatible - you just need to switch the GND and 12V over - I can use the cheaper ULN2803 fitted to the signal control boards for controlling the relays, which reduces the BOM by a couple of pounds per board. Then all I need is a board containing the relays (and possible a home made CDU). This board could even be built first and operated with push switches in order to test the points.