Transbrake "bump box" - Page 1
Forum section: electronics / projects

author: Tempest
author_rank: Newbie
timestamp: May 13, 2020, 08:55:18 pm
content: Hello all,I'm new here, and pretty much signed up just to get some advice on projects I'm working on. Here's the first one I'd like to tackle:I'm big into drag racing, and have a project car which is rife with electronic hurdles. This project is a "bump box" design, which is used to briefly disengage the transbrake solenoid on an automatic transmission under extreme load. If you're curious, I'm sure YouTube has some videos of people "bumping in" to the staging lights - pretty violent stuff.Simply put, this circuit is a timed one-shot that controls a solid state relay. I found the easiest way to build this is using an op-amp and adjustable time constant.The (X) light on the right is a test light for the solid state relay control.While this all looks good on paper (and everycircuit, which I don't exactly trust), I'd like another set of eyes on it as it needs to be robust. First issue is that the discharge time of the capacitor isn't as quick as I'd like. While staging, the transmission is making LOTS of heat, so the faster you can bump into the staging lights, the better. Also, will this design have enough oomph to engage a solid state relay "coil" repeatedly?Any advice is greatly appreciated!

author: John B
author_rank: Frequent Contributor
timestamp: May 14, 2020, 01:17:13 am
content: I wouldn't use that circuit. There are too many unknowns with part tolerances, varying input voltage and the switching mechanism. For a strictly analogue circuit, it sounds like a textbook example for an 555 timer chip configured as a monostable multivibrator. Ie, the switch trigger will result in a fairly accurately timed pulse on the output.

author: Tempest
author_rank: Newbie
timestamp: May 14, 2020, 02:02:22 am
content: How about something like this?Think the 555 will have enough juice to power a SSR directly or do I need a power stage after the 555?

author: John B
author_rank: Frequent Contributor
timestamp: May 14, 2020, 02:47:05 am
content: Something like that. The 555 will certainly have enough output current, though check what input range the SSR needs. Since its an automotive environment, you'll have significant dropout voltage from input protection and voltage regulation. That, and the high state voltage of the 555 is less than Vcc due to its output transistors. You could end up with 8-10V for a high state output voltage.

author: xavier60
author_rank: Super Contributor
timestamp: May 14, 2020, 03:05:46 am
content: Someone asked for something similar on another thread.I'm not certain what this will do when first powered up.I don't know how large a capacitor pin 7 is allowed to directly discharge.D3 can be placed across R2 instead,

author: xavier60
author_rank: Super Contributor
timestamp: May 14, 2020, 03:43:13 am
content: Another concern is debouncing.Another bump could be triggered while the press switch is being released.A capacitor could be put across S1 with a 100Ω series resistor to prevent contact arcing.R4 could also be increased if necessary.The idea is to prevent the 555 from being able to be triggered until S1 has been open for some minimum time.

author: xavier60
author_rank: Super Contributor
timestamp: May 14, 2020, 04:04:51 am
content: Debouncing can also be achieved by adjusting the values of R4, R5 and C2. Mainly increasing R4 and C2.The C2*R5 time constant needs to be short enough so as not to influence the bump pulse.

author: Tempest
author_rank: Newbie
timestamp: May 14, 2020, 04:39:12 am
content: Thank you very much for the guidance - I'll build a prototype and give it a dry run!