When asked by a friend whether it was possible to make a simple circuit to replicate the “bouncing light” effect used by the talking car “Kit” in the TV series Knight Rider, after some experimentation the result was the circuit diagram shown below.
The circuit could have many interesting uses other than in a “talking car”, for example it could provide an electronic “pendulum” effect to a digital clock, or it can be used as an eye-catching warning indicator.
Knight Rider Bouncing-Light Circuit
A timer is formed by U3, a standard 555 timer running in astable mode, with resistors R1, R2, R3, and capacitor C1 determining its frequency. R3 is a variable resistor, used for frequency adjustment and determines the speed of the “bouncing light” effect. The output of U3 is used to control a pair of 4017 divide-by-ten counters, with pulses fed into their clock inputs (pin 14 in each 4017 IC).
When the circuit is initially powered up, Y9 pin of U1 is low, and U1 begins to count as normal, which illuminates the L.E.D.s D16 to D24 one at a time from up to down. As far as Y9 pin of U1 remains low, Q1 applies a logic-high signal at U2’s reset pin, and U2 remains at its reset state.
At the 10th clock pulse after power-up, Y9 pin of U1 goes high and disables any further counting on U1 and it enables counting on U2 instead. When high, Y9 disables clocking on U1, because it is directly connected to its active-low clock-enable input (pin 13 of U1). As far as Y9 pin of U1 remains high, Q1 applies a logic-low signal at U2’s reset pin, and U2 counts as normal. When U2 counts, as the outputs of U2 are connected to the L.E.D.s in the reverse order, the lights seems to move from down to up, in the reserve order.
Once a further clock is received after Y6 output of U2 goes high, Y7 disables clocking on U2, because it is directly connected to its active-low clock-enable input (pin 13 of U2). At the same time, Y7 resets U1 and the whole cycle starts again.
You may wonder what the purpose of using Q2 is. Well, Q2 is used to prevent D23 from being on (lighting) all the time when U2 held at its reset state. The reason is because when 4017 is held at reset, its Y0 output remains high. Without using Q2, D23 would always be on, during U1’s counting cycle (when U2 held at its reset state).
Construction and use
For making construction easy, we have designed a small PCB. The only thing you have to do to make your Knight Rider LEDs circuit, is to solder all the components on the provided PCB. It is essential to take some special care when handling the CMOS semiconductors.
Resistors used in this project are all 0.25 watt, 5% tolerance, or better. C1 is a common electrolytic capacitor at 25V, and all other capacitor are low-voltage ceramics. The circuit can be powered from an ordinary 9V battery. This type of battery is usually designated as NEDA 1604, IEC 6F22 and "Ever Ready" type PP3 (zinc-carbon) or MN1604 6LR61 (alkaline). The battery can be connected to the PCB threw a 9V-battery clip.
After making a minor modification on R6’s value, the circuit can also be powered from any source in the range of 5V to 15V. R6 is used as a current limiting resistor to adjust the current at the L.E.D.s to about 17mA when the circuit is powered from 9V. For using the same current at a different voltage, R6 must be replaced with a resistor of about [1000*(VCC-ΔV)/17] Ohms. VCC is the power supply voltage value in volts and ΔV is the characteristic voltage drop of a LED (also in volts). For instance, for common Gallium Arsenide Phosphide (GaAsP) red LEDs (having ΔV of about 2V) you may use a resistor of about 180 or 560 OHMs for 5V or 12V, respectively.
Author: G. Adamidis
Knight Rider LEDs circuits - PCB Artwork and assembly instructions