Negative voltage one shot - 2 usec - Page 1
Forum section: electronics / projects

author: atferrari
author_rank: Frequent Contributor
timestamp: March 28, 2015, 04:53:02 pm
content: I need to implement a one-shot stage to generate a negative voltage pulse (0 to -3V) lasting around 2 usec, triggered by a TTL pin of a micro. Dual PSU (+/- 5V) available.Been considering an LM311 connected between 0 and -5V. I got just one! of those comparators for the week end and I fear to destroy it in any uncontrolled test. Additionally I cannot see how to trigger it with a positive voltage and even more as a one shot.Reading the datsheet got me even more confused.Any ideas about this or any better/simpler?

author: atferrari
author_rank: Frequent Contributor
timestamp: March 28, 2015, 07:34:56 pm
content: Quote from: evb149 on March 28, 2015, 06:26:11 pmSorry evb,Could you show the schematic of what you propose? Not sure I follow you.Gracias for replying.

author: Kalvin
author_rank: Super Contributor
timestamp: March 28, 2015, 08:04:01 pm
content: 1. Optocoupler- the MCU drives the optocoupler's diode and the optocoupler's transistor is connected to negative power supply along with suitable resistor.2. Negative clamphttp://www.circuitstoday.com/wp-content/uploads/2009/10/Negative-clamper.jpg- The negative pulse with 2us width will be generated when the I/O-pin goes from "1" state to "0" and stays "0" for 2us. When the I/O-pin goes back from "0" to "1" the capacitor will be discharged through the diode and it is ready for another pulse.- If the I/O-line's output voltage level is in 3V rangle, the pulse amplitude should be about -3V. For simple pulse amplitude regulation you could use a zener or few 1N4141s connected in series.

author: Marco
author_rank: Super Contributor
timestamp: March 28, 2015, 09:33:17 pm
content: I'd try a 74AHC123 (CMOS monostable) and save yourself some headache ... create a -3 rail with a linear regulator, put the monostable between 0 and -3, feed input B with a RC differentiator, if you don't want retriggering connect Q to the inverted input A.

author: Zero999
author_rank: Super Contributor
timestamp: March 28, 2015, 10:35:40 pm
content: If you're puzzling over you can interface between the MCU and the monostable, just AC couple them, since it's only the edge you need to worry about.

author: Kalvin
author_rank: Super Contributor
timestamp: March 29, 2015, 12:05:25 am
content: Simple voltage level converter if you are using 3V logic: Put a few 1N4148 diodes in series and connect an 1kohm resistor to -5V. Connect the other end of the diode string to the I/O-pin.Simple voltage level converter if you are using 5V logic: Connect a 2N3904 between the 0V and -5V so that the collector is 0V and emitter is connected to -5V through 1 kohm resistor. Connect 10kohm resistor and a few 1N4148s in series, and connect the diode string between the I/O pin and the 2N3904 base.

author: Zero999
author_rank: Super Contributor
timestamp: March 29, 2015, 01:30:39 pm
content: If the 2us pulse can be generated in software then the CD4007 can be used as a level shifter.http://wiki.analog.com/university/courses/electronics/electronics-lab-voltage-level-shifterOr simpler still, a P-channel J-FET