Creating a slight (~500us) delay when a switch is closed. - Page 1
Forum section: electronics / beginners

author: nsled
author_rank: Contributor
timestamp: May 07, 2023, 11:34:32 am
content: Hello, I'm trying to create a slight delay in the voltage rise when a switch is closed, I can see how this would be done if the load and power supply are separate, just put a capacitor in parallel, but how can it be done when the power supply is the load?The device is a flash unit, with a wired "switch". I want a very slight delay when its triggered.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 07, 2023, 12:28:49 pm
content: Do you really want the delay when you activate the switch by hand? If you use some other mean to activate the switch then have the delay before the switch.

author: nsled
author_rank: Contributor
timestamp: May 07, 2023, 12:42:01 pm
content: The switch is symbolic, but it is physically triggered just not by hand. The delay cannot be before the triggering of the switch.

author: Shay
author_rank: Regular Contributor
timestamp: May 07, 2023, 12:52:08 pm
content: If I understand correctly, maybe a capacitor in series to the switch with a bleeder resistor?As soon as you close the switch, it starts charging, which will add some delay to the rise.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 07, 2023, 01:02:36 pm
content: Basically, you are asking to "debounce" the switch with a 500 us delay.http://www.ganssle.com/debouncing.pdfThe image below is from Grassle. There are other versions. Just charge a capacitor to hold the flash in standby, then discharge it through a resistor and your switch to ground to make it flash.500 us is a very short time for debouncing.EDIT: Attachment #2 shows what I tried to describe.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 07, 2023, 02:01:57 pm
content: Quote from: nsled on May 07, 2023, 12:42:01 pmYou can use a transistor or SCR as a switch and then you delay the base or gate signal that coming from the triggering device like a photocell or camera sync contact.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 07, 2023, 05:12:42 pm
content: Depending on the exact circuit of the flash there may be a substantial voltage across those open contacts. Possibly enough to kill a typical "small signal" transistor.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 07, 2023, 05:38:11 pm
content: The voltage across the switch is shown as 3.3V.I suspected something like a pull-low to flash switch. That is, the positive terminal of the switch does not have a direct connection to VCC. Voltage may be high, but current capability might be quite low. In a way, that is good as one would only need to pull to the appropriate "low" (e.g.,TTL or CMOS). On the other hand, charging of the capacitor could be quite slow. I use a MAX16054 to turn on a circuit (https://www.analog.com/media/en/technical-documentation/data-sheets/max16054.pdf). The pullup is about 100k. If the TS uses a 100nF capacitor and a 10K resistor, the time constant is 1 ms. The RC time constant for charging would be 10 ms (i.e., 3.3V/100k = 33 uA), which is probably OK for slow repetitive flash speeds.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 07, 2023, 06:03:45 pm
content: Quote from: jpanhalt on May 07, 2023, 05:38:11 pmI can interpret the opening post as indicating that the unit is powered from 3.3 Volts, not definitively stating that the switch has 3.3 Volts across it. I can also interpret "The device is a flash unit, with a wired "switch". I want a very slight delay when its triggered." as indicating that the switch is not a power on-off switch but is the switch that triggers the flash.The device is referred to as a "flash unit". Although the built-in flash in mobile phones is invariably a LED, I think a flash tube is much more common in add-on units for cameras.Need clarification.

author: nsled
author_rank: Contributor
timestamp: May 07, 2023, 10:25:02 pm
content: The switch has 3.3v across it, it is a >=0.5us LED high speed strobe. Once a bullet trips the switch i want a short delay so the bullet can be photographed away from the switch. 500us is just round abouts in practice it may be 200us etc. In my head I was picturing changing the cap value to change delay. So is what Shay suggested viable? just a cap in series with switch? I know there's certain specifics that need to be done experimentally like what voltage it actually fires at, what inherent delay it already has etc.I know this can be done any number of ways but I'm specifically after the bullet acting on the switch mechanically.