Hi Henk, Can you help me with understanding this part of the circuit?

Pin 13 of the ATMEGA8 is connected to the anodes of the odometer LEDs and also the cathodes of D15 and D16

Can you tell me if I am interpreting this right?

Mode1
1. Pin 13 is made HIGH and that makes the odometer cct work.
2. Pin 23 and 24 reads the pulses recieved

Mode2
1. Pin 13 is made LOW and that enables D15 and D16
2. Pin 23 and 24 are driven high and low to turn D15 and D16 on and off

Hi,

you're almost correct.

Pin 13 high
1. switches both infrared Leds D13/D14 on so that the odometer photo transistors T11/T12 can detect the reflected light from the odowheels
2. this high level has no influence on the odo detector circuit T11/T12 because Leds D15/D16 are blocking this voltage.
The voltage level on the upper side (collector) of T11/T12 is defined by resistors R18/R20 and the amount of reflected (infrared light) from the odowheels shining on T11/T12.
Now this ANALOGE values (not PULSES) are measured by connecting the single internal micro controller Analoge-to-Digital (AD-) Converter alterating to pins 24 and 23 (ADC1/ADC0) and measure the voltages separately.


Pin 13 low
1. the infrared transmitters are shut off
2. both backleds D15/D16 On/Off can now be controlled by switching pins 24/23 as outputs and drive them low or high.
When driven low or high, the internal resistance of both infrared receivers T11/T12 is that high that they do not have any significant influence on the voltage levels. Neither do resistors R18/R20.

To measure the amount of passing odo-fragments per second you will have to go to a deeper level...
1. measure the voltages faster than the time 1 odo-fragment needs to pass the sensor at maximum speed
2. do some calculation to find the high/low transitions
3. use an internal micro controller timer as a reference for counting the amount of transition per time-unit

Henk

Thanks that helps a lot.

I now have got it working and have written code to measure speed in cm/sec
Measuring distance is only a small step now.