I have this Schwinn Airdyne AD-4 bike (aka "assault bike") and it's a great piece of fitness equipment! I bought it seventeen year ago and it was second-hand then! So, it is quite old! But it is still working and it is just as effective for fitness as it was 20 years ago!
The bike has a control unit that can track the number of rotations per minute the wheel spins. From that it computes the distance and calories consumed. It also tracks time. All these info are displayed on two alphanumeric LCD.
Recently, one of the LCDs died on me. So, it just serves as an excuse to try to improve this bike. The end result should perform the same functions but also be able to upload some data to Internet. This way, I can track how many minutes/calories/distance I did on the bike and see if I improve over time.
The sensor seems to be a reed type. The wheel has a small magnetic disc on one of the spokes and every time the disc is in front of the sensor, the controller reads a LOW voltage.
The main controller is a NEC D7503G - a 4-bit controller. It is the first time I see one (I might have seen others, but I didn't know that they were 4-bit ones)! This controller has an 8-bit timer/counter, a LCD driver, two internal and two external interrupts! It is surprising that with such humble features, they were able to build a working device that lasted for more than 20 years! And it still runs very well - the only problem is the LCD!
I will do my best to keep this PCB intact so that it could be reused (just in case).
In the second post I will describe the new hardware and software and hopefully test a working unit!