Friday, March 2, 2012

Punk Consoles, Lap Timers and IR Transmitters, The 555 Timer

The Personal Lap Timer Infra Red Transmitter has now passed daylight testing - with the aid of a matt black hood for the IR receiver. 

No false laps detected and all genuine laps captured ! 

I know I am late in discovering the 555 timer and the even nicer 556 timer. The web is full of simple, useful circuits using these two chips. Some example applications for RC Include -

As a switched LED Driver, this allows you to drive more LEDs by switching between LEDs so fast that it appears they are all on all of the time, but your power source only has to power some of them at any one point in time !

As an LED Flasher for emergency or track vehicle lights, the same chip is able to switch tens of thousands of times per seconds or once every 10 or more seconds and anything else in between, this makes it ideal for warning lights, police lights etc.

My favorite application of the 555 Timer is the 'Atari Punk Console', look it up, its a nasty sounding little square wave generator that acts as a very crude synthesizer, very little practical use for anything, but its got a cool name and a cool sound.

The 555 is great, but the 556 is so much nicer, for a start its two timers in one package, the pinout is also easier to work with. On the 556 one timer is available through the left side pins and the other through the right side pins this makes bread boarding so much easier.

Personal Lap Timer Transmitter Version 1.0 and 2.0

On the left is version 1.0 using two 555 Timers, version 2.0 on the right uses a single 556 Timer. On the right, you can see the white wire across the IC, this is connecting the output of the control timer on the right side of the chip to the reset pin of the IR Oscillator timer on the left side of the chip. This turns the IR Signal on and off at a frequency we can read on the receiver side, we examine the frequency to confirm that this is a lapping car and not environmental noise.

The current version of the personal lap timer transmitter uses the two timers provided by a 556 Timer IC. The first timer (the IR Signal oscillator), is configured as an astable generating an on/off pulse at 38Khz. This is the frequency required by many popular infrared decoder ICs. The output of this timer drives an infrared emitter LED through a current limiting resistor.

The IR LED has a very tight focus which is great for lap timing. If your looking to build something similar pay attention to the 'view angle' or the equivalent terminology that the data sheet uses to describe the effective signal angle, for lap detection you want as small an angle as possible, for infra red remote control a much wider angle is more useful. 

The second Timer is also configured as an astable, but oscillating at a much lower frequency, I am currently testing 1Khz and 100 Hz versions. The output of this timer connects to the reset pin of the first timer, this causes the infrared emitter to transmit a series of pulses .5ms or 5ms long. On the lap timer side I am looking for pulses in this range, this helps to separate environment noise from actual laps.

If this stage of testing proves successful I will try working with the duty cycle and frequency of the control timer to see if it is possible to create a scheme for determining unique identities for upto 8 cars using a cascade of two or if need be three 555 timers.

Rather than me repeat the excellent work of others I would suggest that anyone interested in more information should have a read through the following links -

Background on the 38Khz IR signal and the ICs Used to detect it see the following link - also take note of the very simple IR Test circuit using just batteries, detector and an LED.

There is a nice background on the 555 timer and its different uses and configurations here -

There are also a several online 555 Timer calculators that will allow you to find the component values you need to produce a given frequency or period, use your favorite search site and look for '555 calculator'

My current circuit uses -

Control Timer - 
R1 670
R2 8K (3.3 + 4.7 in series)
C1 .1uf

IR Oscillator
R1 220
R2 1K in series with 1K potentiometer to give adjustment from 1K to 2K
C1 .01uf

These values are in part determined by what I had available so just use the calculators to work with what you have.

Finally I should mention that the 555 Timer can generate a lot of electrical noise in your circuits so be sure to use decoupling capacitors if its going to share power with anything else.

Duane B


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