Knowledge Base

By Rick Hormigo

A safe way to track lines is the use of an IR transmitter and a couple of phototransistors that keep the reflected IR light from a line.

The block diagram proposed has 4 main parts (Figure 1)

The IR transmitter should be pulsed by one high frequency, high current square-wave generator. Depending of the infrared TX-RX LED’s efficiency we could feed the transmitter with more or less current. The good thing is that if you apply a small duty cycle, the current that the IR transmitter can support is bigger. Since;

When the duty cycle is equal to a 10%, the average power is a 10% too respect to a 100% or DC power. The limit of the IR transmitter is proportional to the maximum power and the instantaneous IR light generated is proportional to the current peak. This means that we can obtain until ten times more IR energy radiated with the same IR transmitter using a 10% duty cycle. The limits in the minimum duty cycle, best convenient frequency, and maximum current respect to the duty cycle are in the manufacturer data sheet. For most of applications the oscillator 555 that include duty cycle set point and current amplifier could be a good alternative.

Figure 1

The amplifier should be designed with sensitivity proportional to the signal generated by the transmitter. The more instantaneous current in the transmitter, the less sensitivity necessary to detect the signal, and the less noise that can get inducted. This sensitivity should be determined experimentally depending of the surface that reflects the IR light and the distance between the surface and the RX-TX LED’s.

In addition, a high pass capacitance should be set up in the input to avoid low frequency light radiation like standard 60Hz lights.

After amplify the signal a pass-band amplifier should be used to eliminate other interference signals. The worst kinds of interference are natural light (DC frequency) and bulbs (60 Hz). Also could be present Remote IR or auto-focus camera transmitters (30-60Khz). If there are not these last devices the best band for IR transmissions is 40 KHz that is the frequency that normally they use. But if there are this kind of devices a lower frequency must be used to avoid interference risk.

Of course this frequency should be the same that the used by the IR transmitter and depending of how much stable the transmitter is we should use more or less pass band.

When the signal is big and clean enough, this can be easily detected using a chip PLL detector like the popular 567. In order to increase the stability can be used the same generator used for the IR as reference in this PLL. This will keep any drift from the generator.

The output of this chip will change to low when the signal is detected, and depending the power supply used a TTL interface could be necessary in the 567 out to interface it with the computer. Look the data sheet.