bicycle speed circuit

Bicycles are our common means of transportation. If equipped with a bicycle speedometer, you can control the speed of riding. Especially for cyclists
, it is a good way to train regularly, reasonably distribute the speed on the way, and master the fast and slow rhythm.
    The circuit of the bicycle speedometer
is shown in Figure 5-33. This speedometer uses
the proportional relationship between the bicycle riding speed and the wheel rotation
to linearly convert the wheel rotation frequency into a DC
voltage, and takes out the pulsating voltage value and
displays it on the voltmeter. The voltage value corresponds to the riding speed.
When riding, the infrared light detector emits a pulse every time the wheel rotates
once, turning on the transistor
VT2 to generate a down pulse to
trigger the monostable circuit composed of the time base integrated circuit 555
. The faster the riding speed, the shorter the time between the two pulses of the infrared light detector , the higher the number of high-level outputs of the monostable circuit, and the
DC average value is indicated by the voltmeter. The meter scale is expressed in speed unit km/h. The number of rotations of the bicycle wheel can
be calculated based on the wheel circumference. Currently, the commonly used bicycles are 71cm (28in), 66cm (26in), 60cm (24in), and 50cm (20in). , the scale can be set to 4 lines
. Calculated based on a 28in bicycle, when traveling at a speed of 40km/h, the wheels rotate about 2.5 times/s. The values ​​of the timing resistor RP1 and capacitor c2 of the monostable circuit
can be iookfl and o. 47vF.
    In the bicycle speedometer, the infrared light detector can be installed in a special bracket using any infrared pair tube, LED and VT1. The bracket is self-cut with a metal
sheet . The shape can refer to the lock structure, and the front wheel spokes of the bicycle where the bracket is installed are A shield is installed on the vehicle so that each time the wheel rotates once, the infrared
light detector is blocked once. The transistor VT2 uses silicon NPN transistor 9014, with a Lu value of about 120. The time base integrated circuit uses NE555.
Potentiometers RP1, RP2. All use WH7 type trimmer potentiometers, and all other resistors use RTX type 1/8W carbon film resistors. The meter head is of type 91C2.
The remaining capacitors are all CJ11 metallized paper capacitors.