CD4541B Timer - Problem with quick restart due to residual voltage on capacitors - Page 1
Forum section: electronics / beginners

author: Cvijo
author_rank: Contributor
timestamp: July 25, 2025, 10:36:32 am
content: Hello everyone,I've designed a PCB using the CD4541B timer (Datasheet:https://www.ti.com/cn/lit/ds/symlink/cd4541b.pdf?ts=1753335762364&ref_url=https%253A%252F%252Fpro.lceda.cn%252F) configured insingle-transaction modewithauto-reset disabled.Intended operation:Turning ON the rocket switch provides power to the chip and activates its output for 5 minutes.After 5 minutes, the chip output switches OFF, even though power (VDD) remains on (this behavior works perfectly and is exactly what I want).Problem description:When I want to restart the timer quickly by toggling the rocket switch OFF and then immediately back ON, the timer doesn't restart. I suspect this happens because large capacitors (100 µF and 100 nF) on the VDD line keep the chip powered briefly after switching OFF, preventing the chip from recognizing the power interruption.If I wait about 30 seconds before turning the switch back ON, the timer restarts normally. To address this issue, I’ve tried adding resistors from VDD to GND to drain the capacitors faster:100 kΩ resistor: no significant improvement (still ~30 s delay).47 kΩ resistor: reduces the delay to ~20 s.1.6 kΩ resistor: reduces the delay to ~0.5 s, which is close but still not instant. The problem here is that this solution constantly draws ~5 mA, which I'd prefer to avoid.My question:Is there any alternative solution or approach that would allow the timer to restart instantly without needing to continuously draw such current?Master reset pin is not used, and I don't think that could help in my caseThe input voltage is typically 12-14 V.Any suggestions or recommendations would be greatly appreciated. Thanks in advance!

author: ledtester
author_rank: Super Contributor
timestamp: July 25, 2025, 01:36:21 pm
content: One approach would be to keep the chip powered and trigger the pulse with one of the reset pins (either MR or AR).This would involve disabling the pulse that occurs on power up. However this thread at TI's E2E site seems to suggest that that idea won't work as the question is asked:Quotewith the response:Quotehttps://e2e.ti.com/support/logic-group/logic/f/logic-forum/1189772/cd4541b-auto-reset-disable-doesn-t-workNonetheless, I'm curious to know what happens if MR is pulled high during power up -- is a pulse emitted on the output and can a pulse be triggered by toggling MR?

author: CaptDon
author_rank: Super Contributor
timestamp: July 25, 2025, 05:43:56 pm
content: Use a single pole double throw switch. Wire the + power to one end of the switch and - power to the other end. On your timer circuit connect the - lead to the - side of the switch and your timer circuit + lead to the center of the switch. When the switch is OFF your timer circuit will appear to have a short across its input draining the charge from the capacitors back to zero. With the switch set to ON your circuit will power up as it does now.

author: ledtester
author_rank: Super Contributor
timestamp: July 26, 2025, 01:20:03 am
content: Here's a schematic of CaptDon's idea:And let me take this opportunity to plug the C005 timer module which you can get for cheap on Aliexpress...https://westsideelectronics.com/delay-timer-ic/https://www.instructables.com/The-Best-Little-Delay-Timer-in-Arduinoville/

author: Cvijo
author_rank: Contributor
timestamp: July 26, 2025, 02:02:34 pm
content: Quote from: CaptDon on July 25, 2025, 05:43:56 pmTy for your suggestion and this would work great. The problem is i am not able to change the switch. This is installed on the boat with control switches preinstalled, waterproof etc. But this would work id i can change them but control panel is fixed. I have attached 2 types but it is the same on every boat.

author: Cvijo
author_rank: Contributor
timestamp: July 26, 2025, 02:06:25 pm
content: Quote from: ledtester on July 26, 2025, 01:20:03 amHi, thank you for the suggestion, but i need chip that works on 12+ voltage. This chip will require additional ldo or buck converter.

author: Cvijo
author_rank: Contributor
timestamp: July 26, 2025, 02:11:49 pm
content: Quote from: ledtester on July 25, 2025, 01:36:21 pmThank you for helping and suggestion. And yes every power up will triger one loop timer. Problem with MR or AR is that i dont have option to send a pulse it is rocket switch so, people will turn it on and leave it on, and MR or AR will keep output high until MR is bring low.If you brin mr high during power uo nothing heppens, normal power up and if you let it be high all the time output is high too, it will start timer when you bring MR low

author: ledtester
author_rank: Super Contributor
timestamp: July 26, 2025, 03:34:03 pm
content: Quote from: Cvijo on July 26, 2025, 02:02:34 pmHow about having the switch control a double throw relay? The only concern might be if the relay was left energized. Looking at a few relays on Digikey it seems that 12V relays have a coil current of around 20mA - maybe a little less.

author: Cvijo
author_rank: Contributor
timestamp: July 26, 2025, 05:34:49 pm
content: Quote from: ledtesterI was rather thinking to put small p channel fet between vdd and ground, gate is connected to +12 so when i turn the power on mosfet will be off, when i switch the power off, gate will drop and turn the mosfet on to short vdd and ground. But i still think that it will not be instant becouse of capacitors that will keep mosfet gate high

author: ledtester
author_rank: Super Contributor
timestamp: July 26, 2025, 06:05:14 pm
content: A solid-state solution is to use a half bridge:When the input 1B is low, Q2 is on and provides power to the circuit (represented by the motor.) When 1B is high Q2 is off and Q1 is on and connects the + terminal to GND.As a cheap solution it might be possible to use a CD4049 as this half bridge. By paralleling the inputs and outputs you can source/sink several milliamps which might be enough for the CD4541 circuit. Here's an example of paralleling four of the inverters to drive a transistor:Since you don't need to source or sink a lot of current another cheap solution is to use ordinary NPN and PNP transistors. Here is an article on testing them in a full h-bridge:https://www.robotroom.com/BipolarHBridge.htmlGraphs and conclusions are on page 4:https://www.robotroom.com/BipolarHBridge4.html