RaceMaster IV Timer Issues Solved
Posted: Mon Nov 09, 2015 11:26 am
Our club has had a RaceMaster IV finish line timer for many years, and there have been several problems with it over the years. The last 2 years, I managed to put some time aside to figure these out, and I'd like to share my findings here, in case others are having similar issues.
Whisker Switch: I've seen several posts about people complaining about these, and I agree. The whisker tends to get bent throughout the race, making the starting trigger unreliable. I replaced the starting line microswitch with a more robust lever actuated microswitch. I bought mine at our local Frys Electronics store, part number VT16031C2, but you can use any microswitch with a suitably robust lever.
Timer Occasionally Would Not Start: All switches have what we call "bounce", where the internal switch contacts would repeatedly make and break connections during the time that the switch is actually activated. The RaceMaster IV circuit includes an RC filter with the intent of debouncing the switch, and cleaning up the signal before it gets to the microprocessor. However, the filter components were not calculated to account for a bounce time of more than about 200us. The switch I was using had a bounce time of up to 2ms. To fix this, I changed C15 to a 4.7uF, 10V, 1206 capacitor. This cleaned up the signal, so that the processor saw a single, perfect rising edge when the starting line trigger switch activated. As a result, the processor does not miss a starting line activation any more. I also replaced C16 with the same size capacitor. C16 is used in the debounce filter for the reset button.
Timer Automatically Resets Seconds After Starting: Upon first powerup, the timer would reset very quickly after starting, often less than a few seconds. The display would read FOO HOO, and then reset to all 0s. This was nowhere near long enough to run a race. As the electronics were powered on for longer periods of time, the timer would run longer before resetting. It would also run longer if one of the finish detectors was triggered. Eventually, it ran long enough for us to run races. It turns out that the input power supply, as it was used in the mezzanine circuit board where the timers reside, was unstable. Over time, the stability would improve. This was likely due to the extensive use of aluminum electrolytic capacitors, which have a tendancy to dry out over time, and lose their performance. I added a 330uF 35V aluminum electrolytic capacitor to the input voltage (because I happened to have a bunch of those lying around), which stabilized the power, and allowed the timer to run flawlessly. I would recommend a couple of 100uF 35V or higher tantalum capacitors if you can find them, as those will tend to last a lot longer than the aluminum electolytics. Pay attention to the polarity of both aluminum electrolytic and tantalum capacitors.
Whisker Switch: I've seen several posts about people complaining about these, and I agree. The whisker tends to get bent throughout the race, making the starting trigger unreliable. I replaced the starting line microswitch with a more robust lever actuated microswitch. I bought mine at our local Frys Electronics store, part number VT16031C2, but you can use any microswitch with a suitably robust lever.
Timer Occasionally Would Not Start: All switches have what we call "bounce", where the internal switch contacts would repeatedly make and break connections during the time that the switch is actually activated. The RaceMaster IV circuit includes an RC filter with the intent of debouncing the switch, and cleaning up the signal before it gets to the microprocessor. However, the filter components were not calculated to account for a bounce time of more than about 200us. The switch I was using had a bounce time of up to 2ms. To fix this, I changed C15 to a 4.7uF, 10V, 1206 capacitor. This cleaned up the signal, so that the processor saw a single, perfect rising edge when the starting line trigger switch activated. As a result, the processor does not miss a starting line activation any more. I also replaced C16 with the same size capacitor. C16 is used in the debounce filter for the reset button.
Timer Automatically Resets Seconds After Starting: Upon first powerup, the timer would reset very quickly after starting, often less than a few seconds. The display would read FOO HOO, and then reset to all 0s. This was nowhere near long enough to run a race. As the electronics were powered on for longer periods of time, the timer would run longer before resetting. It would also run longer if one of the finish detectors was triggered. Eventually, it ran long enough for us to run races. It turns out that the input power supply, as it was used in the mezzanine circuit board where the timers reside, was unstable. Over time, the stability would improve. This was likely due to the extensive use of aluminum electrolytic capacitors, which have a tendancy to dry out over time, and lose their performance. I added a 330uF 35V aluminum electrolytic capacitor to the input voltage (because I happened to have a bunch of those lying around), which stabilized the power, and allowed the timer to run flawlessly. I would recommend a couple of 100uF 35V or higher tantalum capacitors if you can find them, as those will tend to last a lot longer than the aluminum electolytics. Pay attention to the polarity of both aluminum electrolytic and tantalum capacitors.