using a transistor to reset 556 timer - Page 1
Forum section: electronics / beginners

author: ArbuckleRedbeard
author_rank: Newbie
timestamp: October 29, 2024, 11:21:05 am
content: Hello all,I've been playing with electronics for a while (usually trying to fix broken appliances etc) but I have decided to build a circuit to control a linear actuator. I wish for the actuator to be controlled with a toggle switch (in/out) but only for about 10 seconds either way. I want the output from the outward action to cancel / reset the inward action and vice-versa, so the in/out arn't fighting each other..Just using a toggle switch will stop once the button is release, so I wish to incorporate a timerI have tried using a npn transistor to drop the reset voltage.. do you have any suggestions to help make this work?I've been using tinkercad to simulate the circuit.I'm probably going about this completely the wong way..John

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2024, 11:52:48 am
content: The idea of using a transistor to pull-down the reset pin should work.I have no idea whether what you're doing will work or not, because I don't have time to decode your graphical layout into a proper schematic. If you post a propper schematic, with symbols for the components, not graphics, and preferably the pins of the 556 labeled up, or it drawn as two 555s, then I'll be able to assist.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 29, 2024, 11:58:36 am
content: Quote from: ArbuckleRedbeard on October 29, 2024, 11:21:05 amYour schematic is ... not normaland so more difficult to read.Think a moment. Who remembers what are the functions of 556 pins in the order they are around the IC. If at schematic you use symbol with in/outs named then you can understand it.During reading schematic to understand it you don't care about pin numbers - they have no influence on circuit working. Only when you start to assemble it you need that information.I see that you as the output want to use relay and as you write you want this relay to connect to one side for 10s and then stop being connected and in the other situation to connect to the other side for 10s and then stop to be connected (if I understood well what you have written).The problem is that relay have no state "stop being connected". Its 'Common" pin is always connected with NC (normally connected) or NO (normally open) pin (except ms times when it changes state).

author: ArbuckleRedbeard
author_rank: Newbie
timestamp: October 29, 2024, 12:28:10 pm
content: Thank you both for your very quick repliesI've attached a schematic of the circuit, I hope this is clearer.I'm trying to use timer A to push out the actuator, and timer B to pull it back by reversing the contactcts in the relay (so one relay provides power, the other direction) so the 2 timers are not identical, timer A only energises 1 relay, timer B energises the same relay and the reverse direction relayJust thought.. the linear actuator is 12v, I havea 12v supplyJohn

author: PGPG
author_rank: Frequent Contributor
timestamp: October 29, 2024, 01:19:00 pm
content: Look at Fig 7 (and others) how a typical schematic with 555 should look like:https://www.diodes.com/assets/Datasheets/NE555_SA555_NA555.pdfFew questions about your schematic:1. Why K2 NO goes down to then go back up instead of being connected directly? Even such small jump down makes reading more difficult.2. M2 is connected only to K5 so why we need to follow its wires in the mess under everything instead M2 being just placed on the right of K5?3. Why the wire through R3 makes a coil?4. What is the purpose of D2, D5? I didn't even tried to follow their connections.

author: ArbuckleRedbeard
author_rank: Newbie
timestamp: October 29, 2024, 09:09:56 pm
content: Hi, the schematic was automatically generated from tinkercad.. I've just drawn up a diagram of the circuit by hand.. I hope this helps!

author: PGPG
author_rank: Frequent Contributor
timestamp: October 30, 2024, 01:20:12 am
content: Quote from: ArbuckleRedbeard on October 29, 2024, 09:09:56 pmYes it helps.You expect 12V at 556 outputs to drive relays.The first thing that catches your eye:1. I suppose these are 12V relays so why you drive them through resistors.2. You will not get 12V at 556 outputs as they are shorted to your supply '-' by base-emitter transistor junctions (b-e is like diode).For the next step of correcting schematic - do the 556 output current is enough for 2 relays?I am not saying that it is not enough - just need to be checked.I didn't looked how timers are used (it is 2:30 here - the highest time to bed).

author: Zero999
author_rank: Super Contributor
timestamp: October 30, 2024, 09:53:11 am
content: This circuit will destroy the transistors and possibly the 556. There are no base resistors, to limit the current through the bases and there need to be back-EMF diodes across the relay coils, not in series.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 30, 2024, 10:31:35 am
content: Next day thoughts...My intention is to point errors in design and let you find the solutions (I don't want to design it for you).Your intention of using these transistors is to realize this:Quote from: ArbuckleRedbeard on October 29, 2024, 11:21:05 amBut think a moment. Here is logical error.Will your button be able to start timer 1 while it is forced into reset state by timer 2 being currently on?If you have this schematic draw in any software tool (I use KiCad) you will be able to correct it without need to draw the whole schematic once more.Quote from: Zero999 on October 30, 2024, 09:53:11 amI left it as the final touches closer to the end of design.OP's serial diodes are not to protect anything but to drive one relay by two timers and the second by only one of them.

author: ArbuckleRedbeard
author_rank: Newbie
timestamp: October 30, 2024, 06:22:15 pm
content: I haven't tried vicad, but I will... It looks quite comprehensive! I will make do with tippex for now! New circuit attached.I will have a go at creating the updated circuit and get back to you 🙂

author: PGPG
author_rank: Frequent Contributor
timestamp: October 30, 2024, 07:07:12 pm
content: You learn fast.Driving transistors.Transistor have a parameter called beta - it is Ic/Ib (collector current to base current). To simplify you can assume beta=100. So to allow transistor to have Ic=1mA you need to provide into its base 0.01mA. If both collector and base are operate with resistors powered from the same voltage it means that Rb/Rc can be about beta.If you want transistor to work as a switch you supply to its base 5x or 10x higher current than enough one.So in your circuit if you have Rc=10k you can have Rb=1M maximum, but to ensure full saturation (being really ON) you give it 10x higher current so Rb=100k.In your circuit Ic is about 1mA but you drive base with 10mA - 100 times too high current.About the problem that transistor keeps timer in reset so you can't switch it on.Resetting the other timer should not be done by first timer being started, but by button being pressed. I think reset signal can be just the same as TR for the other timer. And you will not need transistors500Ω resistors looks for me too low. Do you need 24mA current in your buttons. 2k .. 10k would be good, I think.Too low current is not good for some kind of contacts but human operated contacts rether not have a problem of loosing contact with time because of too low current as they are cleared by being pressed.

author: Zero999
author_rank: Super Contributor
timestamp: October 30, 2024, 10:07:32 pm
content: It's known as a race condition.Here's an example showing a better way to draw the schematic. I used LTSpice, but haven't simulated it.Note how the positive power supply rail is all at the top and the ground symbols for 0V, which is connected to the negative of the 12V power supply. It also makes it easier if the relay is drawn with separate contacts.Why not simply use the trigger pulse of one timer to reset the other, rather than using transistors?

author: ArbuckleRedbeard
author_rank: Newbie
timestamp: November 03, 2024, 10:46:57 pm
content: Thans for this, this is eactly what Im trying to do!You know, I think I may be an idiot.. Your suggestion to use the trigger is too simple, Im overcomplicating things!!I will try thisJohn