Launch Control Help !!

Hi , i found this circuit on the net .. (https://code.google.com/p/ecureverse/wiki/LaunchControl) ...... So my question is how does the hysteresis work , and also how can i make it faster because its little slow for my need , i changed the VR1 with 150K pot, VR3 with 50K adjust VR2 so that is 3V.My quess its that i am doing something wrong , or its that the fastest that it can go...?

This circuit uses to adjust offset, gain and hysteresis of a tach circuit to switch a relay.

Since VR2 is 2K , setting to 3V out of 7.4V is around midscale so the tap impedance is around 0.8k//1.2k or 0.5 kOhm roughly. This sets the region where the positive feedback resistance VR3 is more sensitive with lower resistance. Since you have changed VR3 from 10K to 50K further away from 0.5k, so you have very little hysteresis and more like a fixed threshold RPM comparator.

Normally Turbo boost has some lag when starting so higher RPM with hysteresis is desirable for passing power.

But if you don't want delay, then the transition to turbo assisted power will be less noticeable.


In any case , you have to decide how you want to manage the turbo with a tradeoff between hysteresis or trigger thresholds for RPM ON to OFF.

https://www.ti.com/lit/ds/symlink/lm2907-n.pdf for details inside chip.

Hi, thanks for the fast reply... i have try this on my car but if i set it to 4000 rpm it drops to 3900 and it kicks again ... but i want to drop down to about 3950 so i get proper boost.I changed VR1 to 100K so i can get rpm range from ~2800 to ~5800.I tried changing VR1 and VR3 various values and this kinda worked best.Is it possible to make it from ~1000 to ~5000 so i can use it as flat shift as well?I am new to electronics so this may sound stupid but what is the diference between hysteresis and trigger thresholds?

Is it possible to make work in that range or should i try another circuit?

Yes. Hysteresis is the +/- offset around a fix threshold.

To get higher acceleration to lower the threshold to enable boost and conversely deaccelerating quickly raise the threshold for cut out. , but that requires more advanced awareness of feedback systems with AC filters to modulate the DC threshold. beyond my ability to explain here, unless I was paid to do the design.