- \n
- No?\u00a0 Go to step 1<\/li>\n
- Yes? Done!<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n
I wanted a small MOSFET that would comfortably switch a low current 12 volt load, triggered by a signal that sits at around 3v most of the time, and drops down to something approaching zero-volts momentarily when the drive head steps.\u00a0 In particular I was looking for a device with a switching threshold voltage between 1v and 3v – any lower than this and there’d be a danger that the step pulses wouldn’t actually trigger the transistor.\u00a0 I settled on 2 devices to try – 2N7000 and BS270.<\/p>\n
From the Googling I’ve done since starting this project, I’ve learned that piezo sounders don’t like having a constant voltage across them in the long term – so my goal for the revisited circuit is to have zero potential across the sounder except when a step pulse is happening.\u00a0 The obvious way to achieve this is to have the sounder in series with a resistor, from VCC to GND, with the MOSFET across the sounder.\u00a0 Since the step signal is active low, this should work in theory – but in practice it doesn’t!\u00a0 The reason is that the step pulse is too narrow (nominally 1\u00b5s, though in practice not quite that brief) to provoke a response from the sounder.<\/p>\n
To get around this, I chained the MOSFET with a good old BC547, with its lengthy storage time measured in the hundreds of microseconds.\u00a0 This significantly widens the pulse and makes it audible.<\/p>\n
![\"GotekSounder2\"](\"http:\/\/retroramblings.net\/wp-content\/uploads\/2014\/04\/GotekSounder2.png\")
<\/a><\/p>\nI’m still “rat’s-nesting” the circuit, but this simplified version is significantly neater than my first attempt!<\/p>\n
![\"2014-04-20](\"http:\/\/retroramblings.net\/wp-content\/uploads\/2014\/04\/2014-04-20-23.50.04.jpg\")
<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"The circuit I cobbled together a few weeks ago to add sound support to the Gotek floppy emulator worked up to a point, but it was ugly as hell, and suffered from occasional stray oscillations.\u00a0 H.M. – the mastermind behind … Continue reading