I was shooting the breeze one day about the good old days with some friends of mine, and we were reminiscing about our car days. Back then I was the proud owner of a 2.0L Ford Mustang, and I was mentioning about a circuit I made during college back in the early 90's that duplicated the Knight Rider scanner in the front of the car also known as a Larson scanner named after it's creator. Having been
re-inspired, I set out to find the old schematic I had drawn. But alas, it was not to be found no matter how many times I looked in the same places. But, I did remember enough to scratch it down again.
The following is a schematic, description, and outline of how it works. This circuit could be easily implemented with a modern microcontroller with almost no parts save a few resistors, led's and the micro of course, but for nostalgia purposes, I've recreated it using logic only as I did in college.
The block diagram of the circuit is quite simple. It all starts with a clock, in this case using a 555 timer in astable mode. This then feeds an up/down 4 bit counter. Which in turn is decoded by a 3 to 8 line decoder. Each output of the decoder then controls an LED with fade effect. Finally a J-K flip flop controls whether the counter counts up or down.
At the beginning is U1 - a 555 timer chip, the go to chip for simple timing applications. The key components are R3 - 1.7K, C2 - 25uF, and R2/R4 - 100 ohm + 10K potentiometer. Using the pot, the frequency range is f=1.44 / C2 * (2 * R2 + R3 + R4)
In this case R2+R4 can range from 100 to 10,100 ohms. Which gives us: Freq(low) = 1.44 / 25uF * (2 * 1,700 + 10,100) = 4.2 Hz Freq(high) = 1.44 / 25uF * (2 * 1,700 +100) = 16.5 Hz
The next stage is the heart of the system, the counter. Here I am using the 74LS191, 4 bit Up/Down Binary Counter.
Most of the pins associated with the counter are not used and left open. Only the Up/Down(U/D), Counter Enable (CTEN), Clock(CLK) inputs and Qa to Qc outputs are used. Because the CTEN is tied low, the counter will always count while the clock is running, and the direction will be determined from the J-K flip flop 'not Q' line.
After the counter is the 74LS138 3 line to 8 line decoder. This will take our binary count and activate the LED associated with it. It too is wired to be active all the time.
The last stage in the pipeline is the LED and decay circuit itself consisting of Q1, R1, ,R5, C1, and LED1. This is repeated 8 times, once for each Yn output from the decoder.
Since the outputs of the decoder are active low, I needed a way to charge C1 when the line goes LOW. Using a PNP transistor here gives me that option. When the base of the transistor goes LOW (when you want that LED on), current flows through the base turning on the transistor. The quick and dirty LED current calculation is:
I(LED) = Vcc - LED forward voltage / R1
= 5 - 2 / 220
= 13.6 mA
Since the base current = 4.3/4700 = 0.91mA, multiply this by the gain of the transistor, in this case approx. 100, yields a collector current of 91mA. This is 7 times the LED current, thereby ensuring that the transistor is saturated and virtually all 5 volts will be applied to the capacitor.
Once the base becomes HIGH, the transistor will be "OFF". At this point, the charge stored in the capacitor begins to flow through R1 and the LED. The capacitor will bleed off charge at a given rate which is what gives us the fade out effect. You can change the fade time by increasing or decreasing the capacitor value. Additionally, you can alter the brightness of the LED by changing the resistor in series with the LED ensuring that the LED current does not exceed the maximum forward current of the LED.
The final part of this circuit is the feedback control from the 74LS76 dual J-K flip flop. The /Q of the flip flop goes LOW when ever J=1 and K = 0 and /Q goes HIGH when J = 0 and K = 1. Both of these conditions occur depending on which end the count is at and causes the counter to switch directions.
Finally, this circuit could easily be expanded to 16 LED's with the use of a 74LS154, 4 line to 16 line decoder and additional LED stages. Simply wire up the remaining counter line (Qd), and attach the K line on the flip flop to Y15 instead of Y7.
And there you have it, a functional old school Knight Rider/Larson scanner made from discrete parts. If you would like to see a scanner completed on newer microcontrollers let us know!
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