Automotive circuit - Page 1
Forum section: electronics / beginners

author: Shent
author_rank: Newbie
timestamp: May 05, 2020, 11:04:50 am
content: Hi, I am trying to modify a drone camera to use as a dash cam but in order for it to work I need to automate the push button that starts recording. The circuit used for the remote controller just employs a series of resistors, whatever resistance is activated via button corresponds to an action ie record, navigate menu's etc. The camera circuit will power up via the 12v battery and will power down automatically after 3 minutes; however i need the recording to start and stop with the ignition. I can wire whatever is needed into the ignition ciruit but have no idea what to look for as the resistor needs to be connected then disconnected at turn of ignition key then activated and disconnected again when the ignition is turned off, can someone help please?CheersMark

author: Shent
author_rank: Newbie
timestamp: May 05, 2020, 11:07:34 am
content: Just thought I would add that I have an arduino starter kit coming as I wanted one and though would need to employ something like this for the solution.

author: Ian.M
author_rank: Super Contributor
timestamp: May 05, 2020, 01:59:53 pm
content: Do you require a momentary 'button press' or does it need to be 'held down' all the time the ignition is on?The latter is easy - a small 12V signal relay would do the job.If you need the former, you can probably get there by putting the relay coil in series with a large electrolytic capacitor. Depending on the minimum load on the ignition switched 12V supply, the relay will probably pulse again when the ignition is switched off, so if you don't want that, put an anti-parallel diode across the relay coil so when the capacitor discharges again it doesn't operate the relay. Experiment with the capacitor value to get a suitable on time contact closure for the 'button press'N.B you *MUST* use a signal relay with gold plated contacts, preferably a hermetically sealed reed relay, as the circuit you are switching doesn't have enough voltage and/or current to break through the oxide film (or any other contamination) that an ordinary power relay's contacts will develop over time.Arduinos are great for simple voltage monitoring, digital sequencing and control tasks, but their external power input and on-board regulator cant reliably withstand the harsh transients typical of an automotive DC supply. If you use one, you *WILL* need an automotive grade power input protection circuit and external pre-regulator.

author: Shent
author_rank: Newbie
timestamp: May 05, 2020, 08:14:07 pm
content: Thanks for the response Ian. Yes after doing a little bit of reading that looks like it would work although i dont understand how it would pulse again when the ignition is switched off. Ignition charges the capacitor and allows 12v to pass to the relay effectively pressing the record button once. Once capacitor is charged power would cut to the relay so the switch would be be open again simulating a depress of the switch. I am presuming that the capacitor is wired up before the relay in the circuit, will it just dump 12v out to the relay once the ignition is turned off?

author: Manul
author_rank: Super Contributor
timestamp: May 05, 2020, 08:21:25 pm
content: Is the original physical button low side or high side? One leg of button should go to ground or V+, which one?

author: Ian.M
author_rank: Super Contributor
timestamp: May 05, 2020, 09:04:49 pm
content: When the ignition is switched off, all the other loads on that circuit will pull the switched supply down, and if they do so fast and hard enough, as the capacitor in series with the relay coil initially tries to maintains the voltage across itself, the relay will get -12V across its coil if it hasn't got an internal anti-parallel diode, and thus will pull in again. However if the other loads are light and one or more of them has significant input capacitance, the rate of fall of switched ignition supply voltage may not be enough to develop enough voltage across the coil to pull the relay in before the series capacitor discharges.

author: Shent
author_rank: Newbie
timestamp: May 05, 2020, 09:06:01 pm
content: Neither as far as I know. The original switch is just a positive wire that runs through a bunch of switches connected to resistors. The voltage that passes through the switch (pictured) indicates which button has been pressed.

author: Manul
author_rank: Super Contributor
timestamp: May 05, 2020, 09:12:59 pm
content: All these switches have a common pin. And this common should go to ground or V+ on the camera.

author: Shent
author_rank: Newbie
timestamp: May 05, 2020, 11:22:48 pm
content: I will have to figure out how to test for this Manul. Ian I have attached 2 pictures of what I think you mean when you state that the capacitor must run in series, example 1 and 2, I have never seen a capacitor run like i have drawn in example 2 but i am very new to this.

author: Ian.M
author_rank: Super Contributor
timestamp: May 06, 2020, 12:02:14 am
content: #1 has the capacitor in parallel with the relay coil. It wont give a momentary action.#2 has the capacitor in series as required. If the RC time constant of the coil resistance multiplied by the capacitor value is long enough, the relay will pull in briefly when the ignition is switched on. Try a 1000uF 35V cap and be prepared to use a higher value capacitor if the relay doesn't pull in for long enough. It helps to look for a relay with a higher coil resistance - if its under 100R it will be difficult to get it to pull in for more than a small fraction of a second, if at all! The capacitor rated voltage is so much higher than the normal supply voltage to withstand the transients an automotive supply is known for.

author: Shent
author_rank: Newbie
timestamp: May 06, 2020, 11:40:03 am
content: That is fantastic Ian, thank you. Capacitor in the basket just struggling to find a relay that is non latching, signal, and above 100 ohms that will take amps suitable for automotive but I'll find one eventually. Once I have tried it this way I might also go the arduino route if possible for control of accelerometer etc. I have a 5v usb on my bike now so could power it with that so just need to find suitable relays and a way to reduce the voltages and amps for the signals into the arduino.

author: Ian.M
author_rank: Super Contributor
timestamp: May 06, 2020, 12:32:43 pm
content: The relay contacts only need to handle a few mA. You do *NOT* need or want an 'automotive' relay as that means big contacts, so high coil power and low coil resistance. Any Form A (SPST-NO) reed relay with a 12V coil >1K and no integrated diode across the coil should do nicely. Here's some at Mouser UK:https://mou.sr/3fpnRoL

author: Shent
author_rank: Newbie
timestamp: May 06, 2020, 12:54:21 pm
content: Thank you again Ian, sorry you have had to literally lead me to water, you have been a huge help.

author: Circlotron
author_rank: Super Contributor
timestamp: May 06, 2020, 01:06:37 pm
content: Quote from: Shent on May 05, 2020, 11:04:50 amThis should do the trick. Diodes are 1N4148 or 1N4001. Capacitor size say 1000uF 35V. May need to try larger or smaller.

author: Ian.M
author_rank: Super Contributor
timestamp: May 06, 2020, 01:22:47 pm
content: Why two SPST-NO relays + steering diodes? If you use a relay with a non-polarised coil, it will pull in as the capacitor charges and again with the coil current reversed when it discharges.However the SPDT (Form C) relay on the left that guarantees the feed to the capacitor gets sharply grounded when the ignition is off is a very good idea.

author: Circlotron
author_rank: Super Contributor
timestamp: May 06, 2020, 01:30:42 pm
content: Quote from: Ian.M on May 06, 2020, 01:22:47 pmYou are so right...

author: Ian.M
author_rank: Super Contributor
timestamp: May 06, 2020, 01:46:52 pm
content: Even if all you've got is a relay with a polarised coil, simply wrap it in a bridge rectifier . . .

author: Shent
author_rank: Newbie
timestamp: May 06, 2020, 10:58:28 pm
content: Thank you both, I have ordered a couple of capacitors and relays as suggested by Ian for now, will have a go at that before i try that circuit diagram as that has gone over my head at the moment but thank you, will report back when i have it hooked up and let you know the results.CheersMark

author: Ian.M
author_rank: Super Contributor
timestamp: May 07, 2020, 12:20:01 am
content: Note: If you put a resistor in parallel with the SPST relay coil, you can reduce the on time without changing the capacitor. If you put a diode in series with the resistor before putting it across the coil, you can get different on times at Ignition switch on and switch off, which may be useful if a long press of that particular button does anything different to a normal press.If for any reason you need separate buttons use separate relays with steering diodes as Circlotron showed in reply #13, but with contacts across separate buttons.

author: johnkenyon
author_rank: Regular Contributor
timestamp: June 03, 2020, 07:58:05 am
content: Many moons ago, I had an external USB hard disc caddy with a separate power supply.It had a pushbutton to turn it on.I wanted it to power on when the external power supply was connected.My fix - wire a 100nF capacitor across the switch contacts. This simulated a key press as the power came on.Won't work in every case - but it should work in the cases where power control functionality is "alive" as soon as power is applied to the unit.