Hi there, I was wondering if anyone can help me design a simple circuit
to drive a switch. The switch has two states, and expects two inputs.
To put the switch in state one, it needs inputs 0V, -5V and to put it
in state two, it's inputs need to be -5V, 0V. So what I'd like to
design is a driver that takes a 0 / +5V signal, and converts it to the
appropriate output state.
Truth Table:
Input Output 1 Output 2
0V 0V -5V
+5V -5V 0V
Now, I thought I could use a hex inverter in a kind-of negative state
(i.e. Vcc = GND, GND = -5V) to generate the appropriate negative
voltage, but playing around in the lab hasn't been so helpful. Can
anyone suggest a way that I can do this? Switching times aren't
important.
Thanks!
Hi,Alex. Even though you didn't state it specifically, I'm assuming
you don't have a +/-5V supply available. If you do, just use an LM393
dual comparator, and place your output pullups to GND instead of the +
supply (note that this comparator, like most, has open collector
output).
|
| VCC
| |
| .-.
| 22K| |
| | |
| '-' VCC
| | +
| | |
| | |\|
| 2.5V o------|-\ 0V/-5V
| | | >-o-----o
| | .---|+/ |
| | | |/ .-.
| | | | |
| Vin | | | |
| o-------)--o '-'
| | | |
| | | ===
| | | |\ GND
| | '---|-\ -5V/0V
| | | >-o-----o
| o------|+/ |
| | |/| .-.
| | - | |
| .-. Vee| |
| 22K| | '-'
| | | |
| '-' ===
| | GND
| ===
| GND
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
However, if you don't have the negative supply, here's one kind of
cheesy way of generating the negative outputs, using a 74C14, 74HC14,
or 40104 hex inverter with schmitt trigger input (view in fixed font or
M$ Notepad):
| ___
| .-|___|-.
| | |
| | |\ | || V1
| .-->|-o-| >O--o-||-o-|<-o--o---.
| | | |/ || | | |
| | --- - --- .-.
| | --- ^ --- | |
| | | | |+ | |
| Vi |\ |\ | | === === '-'
| o-| >O-o-| >O-' === GND GND |
| |/ | |/ GND ===
| | ___ GND
| | .-|___|-.
| | | |
| | | |\ | || V2
| '--------->|-o-| >O--o-||-o-|<-o--o---.
| | |/ || | | |
| --- - --- .-.
| --- ^ --- | |
| | | |+ | |
| === === === '-'
| GND GND GND |
| ===
| GND
|
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
Note that inverters 1 and 2 act as a buffer and inverter of the input
+5V/0V signal. Inverters 3 and 4 are set to be oscillators, with
diodes and caps performing voltage inverter function at the output. At
any given time, either inverter 1 or inverter 2 will be sourcing
current into one of the oscillator caps, preventing that inverter from
oscillating. When that inverter stops, the output load resistor will
pull the charge off the output cap, bringing the output voltage down to
GND.
It's ultra slow, it's clunky, and the output voltage will be a couple
of diode drops short of -5V (use schottky diodes if it helps), but this
might get you where you want to go.
Another option you might be interested in is using a MAX232 or other IC
to get the negative voltage and use a diode to prevent the output from
going above 0V, or use a 4.7V zener if you can live with -4.7V/+0.6V.
Good luck
Chris