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Thread ID: 55276	2005-03-07 03:01:00	Transistor	Dannz (1668)	PC World Chat

Post ID	Timestamp	Content	User
331412	2005-03-07 03:01:00	I have an interface which outputs 0.65 Volts (which is not enough to power an LED) does anyone know of a circuit with a transistor which can use a 9v battery or where i can find such a circuit	Dannz (1668)
331413	2005-03-07 03:24:00	Put your 0.65V through a 1k resistor to the base of a BC547. Connect positive 9V through a 470 ohm resistor to the collector. Connect the emitter to the anode of the LED, and its cathode to the negative. You might need to reduce the 1k ... depending on the gain of the transistor. The transistor will drop about 0.45 or so, the LED 1.7V (red). A high intensity white LED has a higher forward voltage ... around 4, but this will light it. You could reduce the 470 ohm, too.	Graham L (2)
331414	2005-03-07 23:35:00	Ahem . I can't edit that to extinction but no-one has jumped in to cry "nonsense" . ;) There's a couple of problems with that circuit -- and the only excuse is that I haven't been doing any analogue stuff for many years (except in A-D) . Fundamental error: LED in wrong place . . . that would lift the emitter (and the base by an extra 1 . 9 or so volts . No workee . Put the LED in the collector circuit . Probable error: I've always assumed the Vbe of silicon transistors was 0 . 6V, so you would just squeeze in with 0 . 65V . I looked at a few BC54x this morning and the Vbe is about 0 . 69 V . So it's unlikely to go . If it does start to conduct, a 1k would reduce the current so much you might get 1 . 2 or so mA (gain of BC54x around 250) . I also measured an AC128 germanium transistor . Vbe 0 . 186 V --- that would definitely work on 0 . 65 input . ;) Some FETs might work too . . . if they start to conduct at about 0 . 2V (but these are low power FETs with max current of 5-8 mA) . Thinking about it some more, I would use an opamp . Probably something like a 358 (a dual) with a bit of gain in the first amp, and then use the second amp as a comparator (with hysteresis) to drive the LED . But even a 741 as a comparator would work . "Real" comparators often are more trouble than they are worth . . . they can oscillate if they aren't on a proper PCB . Of course I didn't ask the obvious question: what is the source of this 0 . 65 signal? Often a sensor has a transistor output which allows a pullup to avoltage which suits following circuits, with swing between that voltage and ground for on/off .	Graham L (2)
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