Using electronic scheme in the figure below can be made a circuit that allows the user to check the charge of a car accumulator. Checking the battery voltage is done using linear voltage indicator SN16889 (manufactured by Texas Instruments) or MC16889 (manufactured by Motorola). The circuit below allows lighting, depending on input voltage of one or more LEDs. Maximum voltage at which all LEDs light is adjusted using potentiometer P1 to 15 volts. D7 can be red LED indicates a battery voltage too high.
A very simple reaction tester can be made using electronic scheme in the figure below. when the power button Contact is closed, astabil multivibrator built around IC1 produces pulses which are applied to counter IC3. As soon as S2 button was actuated, astabil multivibrator circuit is blocked, the latest LED lights in order to further decoder IC4. Frequency of the astabil multivibrator can be adjusted using potentiometer P1 between 10 and 80 Hz. The circuit requires a supply voltage of 5 volts and a maximum current of about 120mA.