PNP: Base is grounded. When the input signal is 0V, the base and emitter at=20 equal, so the transistor does not conduct. The collector resistor pulls=20 the output to -5V. When the input is 5V, the base is forward biased,=20 the transistor conducts and pulls the collector to ~0V. Problem - If=20 the input signal doesn't get well below .8V, the transistor won't turn off. 2. An LM311 is a voltage comparator, but the output transistor is=20 uncommitted. Both the collector and emitter are present on pins. This=20 allows the output to swing between any 2 voltages that are within the=20 power supply rails. 3. Yes, yes and yes. Did you try a diode? Kerry alan smith wrote: > For some reason I am really struggling with this, should be simple. So l= ets look at the two circuits > > The PNP transistor approach...seems simple but I am trying to understand = how its supposed to work. > > The base is grounded...so the C-E wont be turned on. The collector has a= -5V thru a 4.7K resistor, the emitter has a 1K in series with the input si= gnal. The output is from the collector. So when 0V is on the emitter, the= output is pulled to the -5V reference. When the emitter has 5V on it, it = turns on? Seems to me the base needs to have the input signal, so when 5V = is driving it, it turns on and grounds the output, and when off...it floats= to the -5V? > > 2. Using a buffer. I havent found a buffer that swings -5V to ground.,..= all I have seen are 0 to 5V, other than a LM5112 can swing negative but its= a driver but might work. > > 3. Adding a diode....cathode to the output, with a resistor to ground on = the anode....is this so it become negative biased? such that with a positi= ve input, the diode wont conduct, and the resistor pulls to ground...and wh= en it swings negative the output will then turn on? If I need to clamp to = -5V, would a zener work in this case? > > > > ----- Original Message ----- > From: Kerry Wentworth > To: Microcontroller discussion list - Public. > Cc:=20 > Sent: Wednesday, November 2, 2011 9:09 AM > Subject: Re: [EE] -5V to 0V swing from a +5 input (again) > > Then just swap pins 2 and 3. > But why not run the signal through one of the transmitters, and put a=20 > diode in series with the output? Cathode to transmitter output, anode=20 > to resistor to ground, take signal out from anode. > > Kerry > > > alan smith wrote: > =20 >> what I need is when the input is +5 and need it to be -5 out, and when i= ts low...the output to be low...so basically inverting it. >> >> >> >> ----- Original Message ----- >> From: Kerry Wentworth >> To: Microcontroller discussion list - Public. >> Cc:=20 >> Sent: Monday, October 31, 2011 9:19 AM >> Subject: Re: [EE] -5V to 0V swing from a +5 input (again) >> >> I don't remember the details of what you are looking for, but for=20 >> argument's sake, let's assume you want +5V in to give 0V out, and 0V in= =20 >> to give -5V out. >> >> You could use an LM311 voltage comparator. Use your +8V and -8V (or +-=20 >> 5V) to power it. Tie the output pin (7) to ground and tie the "ground"= =20 >> pin to a pulldown resistor to -5V. Tie the + input (2) to a voltage=20 >> between 0V and 5V (say 2.5V). Apply the signal to the - input (3). >> >> Kerry >> >> >> alan smith wrote: >> =20 >> =20 >>> hello...this goes back to a question I asked a few weeks ago. Basicall= y it was asking about generating a -5V signal from a +5V input. >>> >>> So I settled on a ADM222, old style RS232 driver and sure enough I can = get the -8V drive. But what I need is -5V so thinking....on the charge pum= p -V pin, putting a zener to clamp the voltage to -5V....or....feeding that= -V pin with a regulated -5V, wondering if that might work. >>> >>> However the other problem is, when the input is positive, the output go= es positive, and I need that to be at the reference 0V level. I tried to u= se a pull down to ground on the +V charge pump, but that just pulled it fro= m +8V to +5V. I'm not an analog oriented person so...wondering if there is= some sort of opamp based design that can only allow the negative swing to = go thru, and if its ever positive, keep it at the ground reference. What h= appens if you ground the positive rail on an opamp and just provide negativ= e power? =20 >>> >>> >>> =20 >>> =20 >>> =20 >> =20 >> =20 > > =20 --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .