Both edge detector troubles - Page 1
Forum section: electronics / projects

author: Vakito
author_rank: Newbie
timestamp: September 23, 2020, 12:14:57 pm
content: Hi all, been trying to make a 12v edge detector that pulses when the input goes from high-low or low-high.I have had no luck whatsoever though and the output is uselessly high all the time. Does anybody have any better circuits or even better point me in the direction of an off the shelf circuit I can hook straight up?ThanksPeter

author: Kalvin
author_rank: Super Contributor
timestamp: September 23, 2020, 12:23:40 pm
content: Are you using real CD4077? I mean, if you have substituted the12V18V-tolerant CD4077 with a modern "compatible" device, you have probably fried the IC as the modern devices are typically designed for maximum of 5V power supply.Edit: Fixed the maximum VCC voltage.

author: Vakito
author_rank: Newbie
timestamp: September 23, 2020, 12:34:33 pm
content: I've used a Texas Instruments CD4077BE

author: Kalvin
author_rank: Super Contributor
timestamp: September 23, 2020, 12:50:38 pm
content: TI CD4077BE is a 18V tolerant device, which is correct.How do observe the output? Do you have an oscilloscope?You would probably want to add a series resistor for the other CMOS input in order to protect the device from static discharges.

author: eblc1388
author_rank: Frequent Contributor
timestamp: September 23, 2020, 12:56:18 pm
content: I don't know where you've obtained the above circuit, but its output waveform is wrong!A 4077B will produce the exact logic level output as you've found out, normally high with very narrow negative going pulses when the switch changes state.If you want the output to be normally low with positive going pulses, then use the 4070B instead.

author: Kalvin
author_rank: Super Contributor
timestamp: September 23, 2020, 01:03:26 pm
content: Quote from: eblc1388 on September 23, 2020, 12:56:18 pmYou are absolutely right. The TI datasheethttps://www.ti.com/lit/ds/symlink/cd4077b.pdfcontains also the datasheet for the CD4070BE, and it is very easy to mix the devices. Luckily the CD4077 contains three other gates, and one of the remaining gates can be used as an inverter.

author: Vakito
author_rank: Newbie
timestamp: September 29, 2020, 01:02:52 pm
content: Hi againI have modified the circuit with the extra 10k resistor but have kept the BE chip in place instead of swapping it with a B since it will be used to trigger a 555 monostable, I have simply connected the output to terminal 2 of the 555 via a 10k pull-up resistor.However it is still not completely working, it works as a rising edge detector but not a falling edge detector.I have the monostable set to 6 seconds powering an LED for testing purposesHere's what's happening:Say I quickly turn on and then off the input for terminal 1 of the edge detector, the LED lights for 6 seconds, fair enoughIf I turn on the input for the edge detector for 4 seconds and then turn it off, the LED will stay lit for 2 seconds after I do so, 6 seconds totalIf I turn on the input for the edge detector for anywhere above 6 seconds, the LED will stay lit until I turn the input off after which it will immediately stop lighting.Obviously what should happen is I turn the input on for any amount of time and the LED should light for 6 seconds and when I turn the input off, the LED should again light for 6 seconds.Any advice is apppreciatedThanksPeter

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 01, 2020, 06:06:48 am
content: Connect a voltmeter between 0V and pin#2 of 555 and tell us the voltage in your various switching trials.The voltage should always stay near 12V, no matter what you do to the input switching.