Coolant Warning Delay

This circuit has been developed to give a set delay between the float switch in the coolant tank registering there is a low coolant situation and displaying it on the dashboard via a warning light. 

The standard BMW float switch is closed when there is sufficient coolant in the tank, and open when there isn't. The float switch is connected at J2 - so when the coolant level is OK, it short circuits C1 so it never charges. Should the level drop, the switch opens and C1 starts to charge via either R3, R4, R5 or R6 depending on which one has been selected via a jumper. Once C1 is charged the output of the 555 goes high, and switches the transistor Q1 which turns on the lamp. 

R3 - R6 have been calaculated to give 5 seconds, 1, 2 or 3 minutes delay respectively. Select the one that suits best and stops the lamp coming on from normal movement of coolant.

Veroboard layout (X denotes breaks in the tracks):

A bill of materials can be found here: Coolant BOM