# lm 555

Discussion in 'The Projects Forum' started by sweet1106, Feb 27, 2009.

1. ### sweet1106 Thread Starter New Member

Feb 26, 2009
7
0
hi,

i was wondering how i could use lm 555 timer to measure the time taken by a race car to pass it? i am using ad 633 IC which on modifications can be used as a divider. the length of the race car is the distance traveled by it.

can anyone please tell me how i can display velocity in 3 digits using all these?

i am stuck with how i could input distance and time to the ad 633 ic so that it can divide!

2. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
That is a classic apples and oranges problem. The 555 is mostly a digital device, and you need analog voltages (properly scaled) to represent the values.

I suppose if you had a sample and hold circuit, you could take the quantity of charge on the timing capacitor as the duration of the trigger provided by the passing race car. And if you had some voltage set by a trim pot to represent the length of the race car, you might be able to come up with a value that represented the speed.

But then there is the accurate trigger circuit, both for the 555 trigger and the S&H trigger, and then digitizing the result to display it, and so on. Keeping analog systems tweaked up is no fun. Just obtaining useful voltages is a problem, as they are transient.

I would think that using a microprocessor would make life much easier. All you have to do is note the lap time, and the length of the track will make it quite simple to do time over distance division. And the uC will make displaying the display easier.

But Bill Marsden, our 555 expert, may just come along and prove me wrong.

3. ### sweet1106 Thread Starter New Member

Feb 26, 2009
7
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i really appreciate the fast reply but im sorry to not have said this before! i am not supposed to be using a micro-controller in my project!

4. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
Can you say how you wish to accomplish the car counting - that is, how you will determine the length of time it takes to pass a location? And how that will be an analog voltage?

As an exercise, imagine the race car is 12 feet long, and passes by at 114 MPH. What is the time it takes to pass a point?

5. ### sweet1106 Thread Starter New Member

Feb 26, 2009
7
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i was planning to use an infrared link. i've pasted the info below in blue! i could use the infrared link to activate the timer 555 when the car passes it and it. because when the car passes it, the voltage should go low and the op-amp in the ir link should detect that. similarly when the car is away, the voltage should go high and the lm 555 timer should stop measuring time and once the time is measured, it should reset again.
but what i am not sure of, is , how i could modify the 555 to do all this. how can i reset it?
the question u had (beenthere) has been my question too. about how i could put time and distance in terms of analog voltage.

i've pasted the link for the ad633 info, as well,below. the info can be found on page 5 of the pdf file on the right hand side at the very beginning of the page. this is how this IC will work as a divider. but again how would i input distance and time to them is a question!

Design 1: Low range, Always ON
As the name implies, the sensor is always ON, meaning that the IR led is constantly emitting light. this design of the circuit is suitable for counting objects, or counting revolutions of a rotating object, that may be of the order of 15,000 rpm or much more. However this design is more power consuming and is not optimized for high ranges. in this design, range can be from 1 to 10 cm, depending on the ambient light conditions.
As you can see the schematic is divided into 2 parts the sender and the receiver.

The sender is composed of an IR LED (D2) in series with a 470 Ohm resistor, yielding a forward current of 7.5 mA.

The receiver part is more complicated, the 2 resistors R5 and R6 form a voltage divider which provides 2.5V at the anode of the IR LED (here, this led will be used as a sensor). When IR light falls on the LED (D1), the voltage drop increases, the cathode's voltage of D1 may go as low as 1.4V or more, depending on the light intensity. This voltage drop can be

detected using an Op-Amp (operational Amplifier LM358). You will have to adjust the variable resistor (POT.) R8 so the the voltage at the positive input of the Op-Amp (pin No. 5) would be somewhere near 1.6 Volt. if you understand the functioning of Op-Amps, you will notice that the output will go High when the volt at the cathode of D1 drops under 1.6. So the output will be High when IR light is detected, which is the purpose of the receiver.

6. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
Your IR circuit is good to 10 cm - are you detecting a toy race car? There are other light beam circuits that use better detectors.

Are you expecting to construct this instrument? If that is the case, I would spend quite a bit of time with the AD633 data sheet and any application note you can scare up from the Analog Devices web site.

I would strongly suggest that you get parts and power supply plus a breadboard to construct experimental circuits on. I would spend some considerable time experimenting with the 4 quad multiplier until I felt the results agreed with theory.

As I indicated before, becoming familiar with sample and hold circuits is going to be useful, as the events are quite transient.

To help the project get put together, make a detailed block diagram of the instrument - detector, timer, divider, etc., and then find hardware to accomplish those functions.

7. ### sweet1106 Thread Starter New Member

Feb 26, 2009
7
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yes! i will be detecting a toy race car on the race track that will be built by us.
i do have a supply of the parts, breadboard and all other supplies. that's no problem. i can get any part that i need.

i dont understand what you mean by "I would spend quite a bit of time with the AD633 data sheet and any application note you can scare up from the Analog Devices web site."
As I indicated before, becoming familiar with sample and hold circuits is going to be useful, as the events are quite transient.

i do have a block diagram of how the system should work but i am not clear about the analog inputs that can be sent to ad633 as distance and time.

do u mean to say "ad 633" is hard to use IC? im sorry but i dont really understand what you are trying to say. i would appreciate if u could explain your answer a little more in detail!

thanks a ton!

8. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
Neither am I sure of the "proper" inputs to the 633. That is why I am strongly suggesting that you do some work on a breadboard - or (duh) with a simulator.

I should have thought of the simulator right away, but my wok is very limited, and I can do my experimenting on a breadboard. Anyway, here is a place where you can get Pspice, which is one of the better-known circuit simulators - http://www.electronics-lab.com/downloads/schematic/013/.

There will be some learning involved, but then you will have a tool to use to see if your design ideas are likely to work when built.