Knight Rider Scanner - Model only

Thread Starter


Joined Dec 21, 2011
Ok so I have 2 projects going right now. The first was a Knight Rider scanner for my car and the second is a smaller version of that to hang in my workshop just for looks. So the first one I can't ask about here due to forum rules about automotive mods and applications so I'm hoping I can still get some help here with the smaller version.

Here is the schematic I have gotten to be able to put this together. Rigging up a light bar is the easy part but wiring and controlling it is another story. This schematic is for just that...a control box.

I have read the other threads on similar projects and while the principle is the same I'd like to do something different. I don't want it to just scan back and forth, which is something I can figure out on my own. The 2 things I would like to do in addition to that are 1. A good Comet tail effect and 2. an "all on" effect where all the lights on the bar come on at once and then start scanning. Also I need some help understanding this schematic. I'm still fairly new to the building of electronics. Here is what I can already deduce from the picture...

1. 1 - 74LS00
2. 1 - 74LS138
3. 1 - 74LS193
4. 1 - LM555 Timer
5. 1 - 0.01uF capacitor
6. 4 - 0.1uF capacitors
7. 1 - 0.33uF capacitor
8. 3 - Resistors (470, 1K, 100K)

The questions I have are this...
1. Where are the main power and ground for this circuit?
2. What wattage resistors should I be using?
3. Should the capacitors be polarized, non-polarized...?
4. Shouldn't there be transistors somewhere in this circuit? Possibly at the light outputs?
5. Will this circuit provide the desired effects mentioned above.

As you can tell my knowledge is somewhat limited in this area. Any help I could get would be very much appreciated. Thanks and Merry Christmas!


Joined Dec 26, 2010
Here are some answers to your questions, as follows:

  1. The power is applied between Vcc (+5V for TTL) and the common line, which in this circuit is indicated by ground symbols.
  2. 1/8W resistors should be fine, but you might prefer 1/4W if you are not used to handling small parts.
  3. I expect the low value capacitors shown in this circuit to be non-polarised, and you may not easily find these values in electrolytic capacitors. Use plastic or perhaps ceramic. The symbols shown look like USA style polarised to me, which seems odd, but I don't use those symbols myself.
  4. At 8mA rating, the outputs of the 74LS138 are just about man enough to get a light out of a red LED, but you would not be able to safely guarantee getting the 20mA or so required to get the best out of many LEDs, still less the higher current really high-power devices might need. Driver transistors could be added to do this.
  5. Additional circuitry would be required for the extra effects. Possibly transistor drivers incorporating capacitors could provide an afterglow effect.

Edit: I'm just too slow to be much use these days...

Thread Starter


Joined Dec 21, 2011
Thank You for the help guys! This has been an ongoing project so I'm still just doing my homework. I appreciate the input immensely.

Normally I'd have to close this thread again, it is standard procedure.

Merry Christmas.
On that note...Is it just that I am putting the thread in the wrong forum? I thought the projects forum would be the place for this. I'm afraid I don't yet understand how things work here. Oh time I'm sure I will. Merry Christmas to you as well!


Joined Mar 24, 2008
Nope, it is because we have had more than one person declare their project was not automotive after declaring it was. It is not allowed to be brought up again by that OP, after having declared intent.

Your saving grace is you went to TTL, which is as impractical for automotive as any logic I could have come up with.

Automotive modifications are a major violation of TOS, we do not support them. We will point you to where you can get help, but I am required to close threads trying to do automotive.

We will support troubleshooting (which is not modifications) and some accessories, but it is limited.


Joined Aug 7, 2008
Using a higher gain LED driver [ darlington MPSA14 ], allows a more presice control of the comet tail, R4, C2, selectable from none to full on. The full on here is achieved with Q1 and associated circuitry to control from one driver to several. When power is applied, via SW, the top of R3 goes high long enough to charge C2's [ 2μF ] & hold it a few tenths of a second. If a longer tail is desired , a bias resistor [ 300k-1M ] can be added from base of Q1 to supply V. As used in this project VDD is a battery of 4 AA Ni-MH, cells tapped one down from top for LED's. Ea red driver controls 21 bi color LED's. The green LED's have no tails. Total of 147 LED's.