Never touched a 555...Now i need to. Help please! - Page 1
Forum section: electronics / beginners

author: Richb77
author_rank: Contributor
timestamp: October 25, 2023, 10:11:55 am
content: Hi All.I wonder if someone with more knowledge than me (that's NOT hard) can help me out.I have a pre bought circuit (H bridge) to flick the polarity of a motor back and forth. The only issue is the trigger for this switch is pressing a momentary switch (3mm tactile SMD type).The project I have in mind will require me to power the device on and leave it to do its stuff rotating back and forth.I thought a small 555 timer circuit with an output soldered to the switch pads could replace the tactile switch. But here's my issue. That seems an unusual use for a 555 circuit and I cannot find any schematics or aid in designing one to do that with a 555.I thought a circuit with Pot adjustment for timing and the 555 activates the momentary switch accordingly.HELP. I have exhausted my pathetic knowledge of 555 timers. Stupid I know but I have just never come across them before.Thanks chaps and ladies.

author: Psi
author_rank: Super Contributor
timestamp: October 25, 2023, 10:29:50 am
content: If i understand you correctly, you will probably need two 555 timers.https://www.circuits-diy.com/repeating-timer-using-two-555-timer-ics/

author: iMo
author_rank: Super Contributor
timestamp: October 25, 2023, 10:40:56 am
content: You have to show us the wiring of the switch (around your tactile switch).Otherwise do use a relay at the 555's output (wired as an "astable multivibrator") switching the contacts wired to the switch on/off periodically.

author: Ian.M
author_rank: Super Contributor
timestamp: October 25, 2023, 10:45:11 am
content: You should also give us a link to the H-bridge module you bought. There may be a simpler way to do what you want e.g. disable its toggle section and feed it a simple direction signal generated by a 555.

author: Richb77
author_rank: Contributor
timestamp: October 25, 2023, 10:47:50 am
content: This is the circuit I am using (the polarity switching one). I want to replace the tactile switch on the lower half/centre, which activates the polarity swap.http://Found herehttps://www.aliexpress.com/item/1005005720509835.html?spm=a2g0o.order_list.order_list_main.64.72461802zSetWP

author: iMo
author_rank: Super Contributor
timestamp: October 25, 2023, 10:59:53 am
content: Thus you have to power the 555 off the 5V regulator on that board, the output of the 555 via a protection resistor, say 220ohm, wired to the hot side of the switch (the input of the MCU there). 555's GND wired to the GND of that regulator on that board..A short negative pulse (like 200ms long) off the 555 output (going from 5V to gnd and back to 5V) will do..You have to configure your 555 such it generates a negative 200ms long pulse, repeating it every X seconds, in an "astable multivibrator mode".

author: Zenith
author_rank: Frequent Contributor
timestamp: October 25, 2023, 11:11:43 am
content: I'm not quite sure I understand this problem. I gather that you have an H-bridge circuit with a tactile switch. When you push the button the motor reverses direction and stays that way until you push the button again and the motor again reverses direction..So, you need a pulse of say a 100ms duration to activate whatever stands for the tactile switch, then a pause of some seconds which is variable. In other words a low duty cycle, long period square wave. It should be possible to do that with a 555, or as Psi says, two 555s, one to provide the short pulse, and the other to provide the delay between pulses. It's been a long time since I've played with them. I'm not sure about connecting the output of the 555 directly to the controller. It might be better to use a relay.

author: Ian.M
author_rank: Super Contributor
timestamp: October 25, 2023, 11:53:27 am
content: That looks like its based round an 8 pin STC8F series microcontroller driving a transistor to operate a DPDT relay wired to reverse the motor. The relay isHRS2H-S-DC5V-N.The switch grounds pin 3, with the relay drive output (+ LED D2) on pin 1, and LED D1 on pin 7. Pins 5, 6 and 8 go to the mode jumper pads P3.If I am reading the item description correctly, its supposed to autoreverse, with user selectable forward and reverse times up to a little over 80 minutes (or a bit over 11 hours if you solder the P3 jumpers). Instructions for setting the delay times can be found on Amazon:https://www.amazon.co.uk/Reverse-Polarity-Module-Adjustable-Current/dp/B08NXDWZ9NYou shouldn't need a 555!N.B. the relay is only rated for 2A 24V DC. If you don't have a snubber circuit across the motor to clamp the back-EMF spike to under 24V or exceed approx. 100mA motor running current (as the stall current can be up to 10 times greater, and the'plug' reversingcurrent, double that), you'll burn out the relay fairly quickly.If you were going to use a 555, (not recommended) a small signal Schottky diode (e.g. BAT48) connected between the switch pin and the 555 output, cathode towards the 555, would let the 555 pull the MCU's switch pin low when its output is low, with no need to match the 555 supply voltage to the MCU.

author: jonpaul
author_rank: Super Contributor
timestamp: October 25, 2023, 12:00:00 pm
content: Walt JUNG IC timer "cookbook"https://archive.org/details/ictimercookbook0000jung/page/n5/mode/2upJon

author: pqass
author_rank: Super Contributor
timestamp: October 25, 2023, 01:48:58 pm
content: You didn't state your timing requirement. The 555 is really only good for less than minute timing pulses since longer durations need larger resistor+cap values (megaohms and >100 uF) which makes timing harder to control.Below is a variation of a classic PWM circuit where, given the values chosen, we can vary the pulse repeat frequency from 15s to 80s (left pot) and width of the negative pulse from 0.5s to 1.3s (right pot, although you'll probably just replace with a suitable resistor in your final implementation).From the photo you provided, the tactile button appears to ground the MCU pin. We can take advantage of the 555s pin 7 (discharge) being an open collector and directly tack a wire from either upper button pins to pin 7. LED+resistor optional.You can play with the simulationhere.

author: Zenith
author_rank: Frequent Contributor
timestamp: October 25, 2023, 11:46:44 pm
content: Quote from: pqass on October 25, 2023, 01:48:58 pmWell there are solutions involving 555s, decade counters and avoiding huge values of resistance and capacitance, but it seems like something from a bygone age.None the less, messing about with 555 timers can be most instructive.This seems to be an application begging to be satisfied by a microcontroller board, such as a Raspberry Pi Pico. There are others. The coding involved is not hard. The OP would find plenty of help on here were he to struggle with it.

author: Smokey
author_rank: Super Contributor
timestamp: October 26, 2023, 12:27:52 am
content: Why not just get a DC motor driver board, preferably one with a little micro already on it, and control the motor with that. Not everything needs to be a Rube Goldberg device.

author: Ian.M
author_rank: Super Contributor
timestamp: October 26, 2023, 12:46:30 am
content: Quote from: Smokey on October 26, 2023, 12:27:52 amThat's what the O.P. bought. Its a STC8 MCU running timer firmware, toggling its output to operate the relay to reverse a motor.I found the same board on Amazon with this description:QuoteThose instructions are as clear as mud, but some experimentation should be enough to figure it out.Of course, it would be a lot nicer if it had an ATtiny or similar with support for programming it from the Arduino IDE, and a proper H-bridge IC, but then it would be a lot more expensive than a $2.26 Chinese piece of crap!

author: Richb77
author_rank: Contributor
timestamp: October 26, 2023, 07:55:42 am
content: Firstly, thanks for all the replies and information.I too found the Aliexpress and Amazon descriptions as clear as mud and thought it was just my Mechanical brain not knowing electronics well enough.Some testing has proven things though.This PCB does EXACTLY what I want already. Spins the motor (powered through the board but at different voltage to the PCB circuitry) then after a given time reverses it...and carries on doing so. So I need no 555 timer (although reading the replies it wasn't the right choice).Again. My heartfelt thanks for sharing your knowledge

author: EPAIII
author_rank: Super Contributor
timestamp: October 27, 2023, 07:02:41 am
content: YES!Quote from: jonpaul on October 25, 2023, 12:00:00 pm

author: EPAIII
author_rank: Super Contributor
timestamp: October 27, 2023, 07:13:22 am
content: I'm not sure that we understand just how that board operates. There are a set of three jumpers that allows eight "delay" settings. What's that all about?Does the switch, obviously a momentary, actually do? Does the motor run one way when it is pressed and the other way when released (definite action)? Or does it reverse the direction each time it is pressed (alternate action)? Or ON/OFF? Or what?And then, what is that delay setting all about?AliExpress is more of a crap shoot than you can ever find in Las Vegas.There is probably a processor chip on that board. If you can ID it, one approach may be to reprogram it or toss it and substitute one with your own code.

author: Zero999
author_rank: Super Contributor
timestamp: October 27, 2023, 08:14:03 am
content: Quote from: Zenith on October 25, 2023, 11:46:44 pmI'd recommend the '4060 for low frequencies because it has an oscillator built-in, so only one IC is required.