Why is this 3901 transistor not always triggering (o-scope photo) - Page 1
Forum section: electronics / projects

author: Justin_
author_rank: Newbie
timestamp: March 30, 2022, 02:42:59 pm
content: Hey guys, im helping somebody out with an '01 Miata. The anti-theft they have is starting to die and they need a replacement to keep the car going. Fortunately the anti-theft is not that hard to emulate and somebody had already done the researchhttps://forum.miata.net/vb/showthread.php?t=504294. Ive been following the work from Alcantor with the circuit as follows, my issue is with Q1* DATA Line* Bus = Line between Immobilizer and PCM (Red/Black wire).** Bus RX_INV_PIN* | | c* | R1 /* +----/\/\/\----| b Q1* | \* | | e* | c GND* \ R2* Q2 | b ---/\/\/\--- TX_INV_PIN* /* | e* GND** R1 = 150kohm* R2 = 1.5kohm* Q1/2 = Any NPN Transistor (BC547)** It's a bus with 1200 bits/s, 1 start bit followed with 8 data bit.* Need 7.5 mA to pull down the bus. (40 mA on ATtiny)I ended up using 3901 transistors as that is what was on hand.I also used 100K and 1K resistors respectively.The Problem:When I start the car as normal things work fine.When I start the car while it is on a battery charger things don't work (o-scope)There are also some other conditions that I haven't nailed down that will make the car not start.I've tried scoping out the micros power line (regulated down to 5V) and it seems to remain stable when starting the car. Still, I also tried adding a capacitor, and that made no difference.When the charger is connected there is a bit of AC noise on the line, but the line still comes down to ground and remains there.Im really not sure what is going on here and I am hoping somebody can take a look at the photo and recognise "Oh, that's the such and such effect". While I have done a few projects with arduinos and such im not well versed in the actual operation of components like this.

author: moffy
author_rank: Super Contributor
timestamp: March 30, 2022, 03:12:49 pm
content: Couple of thoughts:1. Do you have a pull up resistor connected to RX_INV_PIN, say 10k. You might be picking up noise.2. The base resistor looks a little large, noise susceptible, perhaps if you place a small capacitor between the base and emitter of Q1, say 47pf, it might solve the problem.

author: strawberry
author_rank: Super Contributor
timestamp: March 30, 2022, 04:00:22 pm
content: DTC114

author: inse
author_rank: Super Contributor
timestamp: March 30, 2022, 05:28:17 pm
content: The bus voltage is not going down to 0, possibly the voltage is sufficient to keep Q1 on.Add a pull-down from base to GND.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2022, 05:35:52 pm
content: Try a B-E resistor, say 10 to 100k, on Q1. A problem is, if the bus pulls down to ~0.5V when low, Q1 will only barely turn off, and quite slowly at that (not nearly slow enough to matter at this baud (~20us), but something to keep in mind for other applications). If the bus is driven by other weak drivers, or say Darlingtons that hardly pull below 0.7V anyway, you'll get a poor logic threshold this way. Or if there's significant voltage drop between your circuit and whatever's driving the bus; which doesn't need to be much (~0.5V), especially under starting conditions (100s A flowing through the steel chassis).The B-E resistor ensures two things:- The B-E junction stores some charge; if left floating, it does eventually turn off, but it takes a while (~20us). Same is true for a high resistance to a marginal "low" voltage. There is also more leakage current with it floating (but this shouldn't matter with a modest value pull-up on its collector).- Driving the base from a resistor divider like this, shifts the voltage threshold up, from ~0.6V to somewhere closer in the middle of the bus voltage range, say 3-6V. So, using a 1:4 to 1:9 ratio would seem ideal here.This does reduce base "on" current slightly, but the "forced" or "saturated" hFE should be more than low enough not to matter (i.e. Ic/Ib ~ 10, even 50 is likely fine).Note that the MCU pin drives low pretty solidly (CMOS switch) -- effectively Q2 already has a ~1.05k B-E resistor. Which isn't ideal for the (5V?) drive it's given, but it's absolutely still going to work for this.Tim

author: moffy
author_rank: Super Contributor
timestamp: March 30, 2022, 06:33:15 pm
content: T3sl4co1l with a B-E resistor makes more sense, and would explain why when the battery charger is on you have problems, the battery charger raises the overall battery voltage at start.