The Do-It-Yourself Race Timer

Tuning The Sensitivity

If you use a phototransistor that is different from the model specified, or if you change the separation distance between the LED and the phototransistor, you may need to retune the circuit to work properly under the new conditions. Tuning usually involves only replacing a few resistors, and can be accomplished in just a few minutes if you have resistors of the correct values and a soldering iron on hand.

This procedure will make references to parts in the two lane version of the timer circuit. All other versions of the timer circuit follow the same basic design, but the parts may be labelled differently. If your timer was built from one of the other designs, use Page 3 and Page 4 of the two lane design to help identify the right parts in your circuit.

Begin by removing power from the circuit. Then disconnect R23 from the output of IC13a and connect it to +5V, but leave the output of IC13a disconnected. This will cause D10 to be turned on at a constant intensity when power is restored to the circuit.

Mount D10 and Q1 in their positions on the track, or in a test jig which allows you to position them with the same separation distance as they would have if mounted on the track. Connect a volt meter across R11.

Restore power to the circuit. Adjust the aim of D10 and Q1 to get the highest reading you can from the volt meter. Make a note of this reading. Place your hand immediately in front of D10 so that it blocks the beam completely. Make a note of the reading on the volt meter. Remove power from the circuit again.

Subtract the two voltages that were recorded. If the difference is more than 2.5 V then R11 has too much resistance. Select a smaller value resistor and replace R11 with the new resistor. If the difference is less than 2 V then R11 does not have enough resistance. Select a larger value resistor and replace R11 with the new resistor.

Calculate the new values for R12 and R13 such that they satisfy the relation R12 = R13 = 135000-(2*R11). Round to the nearest standard resistor value. If this value is less than 10000 then change C7 to .001 uF (1000 pF) and use the relation R12 = R13 = 637000-(2*R11). Replace R12 and R13 with resistors of the value just calculated.

Now that you know the correct values for C7, R11, R12, and R13, use these same values to replace C9, R17, R18, and R19. Do the same for any additional lanes in your version of the timer.

Disconnect R23 from +5V and reconnect it to the output of IC13a. Restore power to the circuit and verify that it operates correctly.

Copyright © 2001 Kristin Hammond

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