Latching Relay Car Immobiliser - Page 1
Forum section: electronics / projects

author: Kasie
author_rank: Newbie
timestamp: October 28, 2018, 03:41:12 pm
content: Greetings All,I've joined the forum in the hope that I could get some guidance on 2 little "projects".First off, I need to apologize for the rather lengthy description.Being a mechanically minded person, (and also not so young and sharp as I used to be), please bear with me as I know very little about electrical stuff and even less about electronics.I have done some "web surfing," and came up with the info shown on the attached diagrams.FIRST PROJECT - LATCHING RELAY.I've built the latching relay and installed it in a VW Golf Mk 1 as an immobilizer. The car's systems are basic, and does not even have fuel injection or any other electronic engine management system. It does have electronic ignition (Hall Sender and controller unit) though.The relays are supplied with switched power from the car's fuse box and interrupts the ignition system - not the starter circuit as indicated on the diagram. It generally works like a charm. When pushing the momentary button, I can hear the relays clicking in / latching.However, on the odd occasion it seems that either one or both relays fall out (if this is the correct term) during cranking.My questions are:1) Would a capacitor and diode across terminals 85 and 86 of either or both relays prevent it/them from falling out?2) Would any additional components (besides a capacitor and diode) be required?3) What size capacitor would be recommended?4) Would there be any detrimental consequences to this modification?SECOND PROJECT - KEEP CAR RADIO ON WHILE STARTINGThis little project is purely an attempt at eliminating a "nuisance factor".All it really is, is a capacitor and diode in the power supply to a standard stereo car radio. No amplifiers, boosters, extra speakers are connected.My questions are:1) Would this work as indicated on the wiring diagram?2) If so, are the values of the components correct?3) If I've missed the pot completely, are there any recommended alternatives?If at all possible, it would be preferable to avoid complicated circuits involving transistors, resistors and other such mysterious items. Having said that, if there are no alternatives, I am willing to attempt building something on a "paint-by-number" basis.Any guidance, assistance, recommendations (or even a slap on the wrist if needed!) will be sincerely appreciated.Many thanks indeed.

author: Ian.M
author_rank: Super Contributor
timestamp: October 28, 2018, 04:31:58 pm
content: The supply voltage can drop as low as 8V when cranking even if the electrical system is in good condition. No practically sized capacitor will keep the radio running for the several seconds it can take to start the engine.A large electrolytic capacitor *may* help prevent the latching circuit dropping out, but care must be taken to manage its charging current to prevent contact burn of the latching relay and/or momentary switch. It probably will need a series resistor to limit charging current + a Schottky diode across the resistor, (anode to the capacitor + terminal) so it can freely discharge into the load.IIRC Mk I Golf battery terminals were fairly crappy pressed steel ones. If they haven't been replaced, you should do so, preferably with heavy duty cast brass ones to minimise resistance. The ground strap probably also needs replacement. Uprating the battery to a higher capacity one is worth doing if it needs replacement - look for the largest one that will fit the battery tray with the same flanged base clamping arrangement to secure it. It will vastly improve cold starting performance and increase battery life by a few years, and will probably also keep the radio and your latching circuit from dropping out due to undervoltage.

author: Kasie
author_rank: Newbie
timestamp: October 29, 2018, 06:35:31 pm
content: Greetings Ian,Thank you for taking the time to respond, it is much appreciated.Regarding the radio "dying" during cranking is something that I'll probably have to live with.The battery terminals have been replaced a couple of years ago, although not with the cast brass ones as you recommended. I will certainly look at replacing them.I will also have a close look at the ground strap/s and earthing points. The car's battery was replaced about a year ago, so it is still ok. When the time comes, I'll definitely uprate the battery.Regards.

author: Kasie
author_rank: Newbie
timestamp: November 24, 2018, 05:13:55 pm
content: Greetings,The matter of a latching relay in use as a car immobilizer refers.I have eventually identified the most likely cause of the relay dropping out occasionally.The problem lies within the car's ignition switch. It would seem that there is a momentary loss of power when the key is turned from the ignition "on" position to "start" position. This is especially evident when the key is turned slowly. The relay drops out before the starter solenoid engages.I have no idea whether this is inherent in the design or due to normal wear and tear.Does anybody know of a way to get around this problem? I'm not sure if adding a simple capacitor/resistor type "delayed power off" circuit would work. There are quite a few of these circuits on the web.Many thanks in advance.Regards.