Input on simple vehicle circuit - Page 1
Forum section: electronics / beginners

author: cmumford
author_rank: Contributor
timestamp: February 17, 2024, 06:22:09 pm
content: I'm trying to put together a simple circuit to control two motorized steps based on vehicle status. Attached is my rough draft. I'm new to logic gates and transistors so have what I think are some basic questions.Some notes on my circuit:The schematic currently has CMOS logic gate components. This is so that I can have a simple schematic for this post. My intention is to use combined IC's like theCD4081B(four channel AND gate), etc.For some reason the sliding door is low (0V) when open and 12V when closed. All other doors are 0V when closed and 12V when open.My questions:Is using logic gates the correct approach?Is using an NPN transistor the right way to switch the flow of current? I believe these are just logic lines to the controller - i.e. the motor current will not be flowing through these lines to ground.Is using an inverter the best way to invert the passenger door signal? I'm thinking of when the vehhicle voltage is above or below 12V. i.e. if battery is 12.8V and the door line reads 0.2V, will the inverter convert this to 12.8V-0.2V=12.6V (or thereabounts)?Should I add anything to the circuit to protect it? I hear that automobile power can be very noisy, spikey, with wildly varying voltages.Very grateful for any input you can offer.

author: Zero999
author_rank: Super Contributor
timestamp: February 17, 2024, 07:07:51 pm
content: The transistors need base resistors to protect both them, as well as the logic gates.What's connected to the outputs? How much current does it use? Is it inductive?How about the CD40107B which can drive 138mA or so, directly: no additional transistors required.https://www.ti.com/lit/ds/symlink/cd40107b.pdf

author: cmumford
author_rank: Contributor
timestamp: February 17, 2024, 08:35:21 pm
content: Quote from: Zero999 on February 17, 2024, 07:07:51 pmThx on the resistors - will look into that.The outputs are connected to a controller for powered steps. I think they are 12V logic lines - nothing inductive. The two wires (one for each side of the vehicle) are ground switched. So floating will retract the steps, and pulling to ground will lower them. I can't find any good information on these, but I figured that if they do draw too much current for a transistor then I will replace that with a relay.

author: jonpaul
author_rank: Super Contributor
timestamp: February 18, 2024, 12:34:50 pm
content: Engineers moto: KISS..keep it simple stupid.1/ Any electronics in vehicle must withstand kV of DC bus transients due to inductive loads like motos and solenoids and alt load dump 75V many J for 100 mS. Need sspecial TAZ, we used MOT MR2525 and MR2535 diodes in 1980s.2/ No need for ANY electronics or logic. A simple relay logic with a few relays can accomplish required OR/AND/NAND/XOR function.3/ Search Relay Logic, invented by Claude hannon in 1930s.Bon ChanceJon

author: Ian.M
author_rank: Super Contributor
timestamp: February 18, 2024, 12:52:57 pm
content: It needs interlocks to prevent step deployment if the vehicle is moving. e.g. for automatic transmission, is it in park?, or for manual, is the handbrake on?Also, what sensors do you have to prevent it breaking the ankles of someone opening the sliding door from the outside?

author: MrAl
author_rank: Super Contributor
timestamp: February 18, 2024, 01:09:36 pm
content: In a word: PNP ---> NPN + Rbase