Moving Car Transit

Thread Starter


Joined Nov 20, 2011

I'm currently in a school project assignment where we are supposed to design a car that passes through stations and upon a passing through 8 of stations, the car will automatically reverse. The car must be able to count the total number of 8 stations, then it passes on to a HEX display and also count down when it reverses. The car must be able to stop upon hitting an emergency button in both directions

We are provided with the following:

1) 74LS32 OR gate
2) 74LS08 AND gate
3) 74LS04 NOT gate
4) 74LS02 NOR gate
5) 74LS00 NAND gate
6) 74LS74 D flip flop
7) 2N3904 NPN transistor

Light Dependent Sensor Circuit

Counter and Display Circuit

Motion Control Circuit

Count Limiting Circuit (Need help with this!)

I have already done the circuits above, all that is left is to create a count limiting circuit board that controls the count, whether the car moves forward or backward, basically it is the brain of the car. I am stuck with the forward and backward signal. We can tap the CLK signal from The Light Dependent Sensor Circuit and also Q0-Q4. I wonder is there any clues for making the signal because my group have tapped Q4 into a multiplexer (74LS157) A/B toggle but the 7-segment can only count from 0-7 and 7-0, not 8. We need to display 8.
Last edited:


Joined Mar 24, 2008
Since this is homework I will move it to the Homework form.

Nice to see someone posting schematics, I suspect you'll get the help you need.


Joined Nov 25, 2009
How is the light signal supposed to control the cart? What event does the light signal flag?

I suppose that by Q4 you mean actually Q3, since the counter has only 4bits. Why do you have a problem with it. You can simply start the count at 1, instead of 0, and have station "8" displayed in the final destination.

Thread Starter


Joined Nov 20, 2011
Things to amend. You guys are right, i made a mistake by saying that the we can tap from Q0-Q4. It's actually to Q3. Sorry!

Our project is kinda similar to this.

So, we need to pass through 8 stations/gantries. The LDR from the Light Dependent Circuit will tick when light is blocked and act as a CLK signal. Then the CLK signal will feed to 7 segment circuit and count from 0 to 8 (MOD 9). After it reach station 8. It will automatically reverse and count down to 0. My group have simultate the counter from 0 to 7 and fronm 7 to 0 but not 8 by using multiplexer.

Thanks! :)


Joined Feb 19, 2009
How many parts of each type in the list are you allowed to use?

Have you drawn a state diagram or logic truth table to describe the behavior?

I'd suggest a state diagram, from any position, usually put in a circle, such as "Start", lines are drawn out of that circle with variables that would change action, ending in more circles.

Essentially, an ugly flow chart with usually criss-crossing lines on the first few drafts.

Once you have that drawn, put together the logic to do what the diagram says.

Or you can make a truth table/Karnaugh map for the logic.


Joined Nov 25, 2009
Please try to be more explanatory in your posts. From what you have written, I guess, that what you have done is use the Q3 output from your counter to select an input from a 2-to-1 MUX, outputting a 0 or 1, depending on Q3. I also guess that your problem is that even though this way your cart will reverse immediately after reaching the 8th gate (is this desirable?) your count also switches immediately to 7 again.

This is not a circuit fault, it's the way it is designed. What is the behavior you want the cart and the counter to have? Do you want it to go past the 8th gate for a little, while having a count of 8 and then reverse back? Do you want it to stop under the gate while displaying 8?
It's all a matter of knowing what you want your circuit to do.