The 555 timer has low cost and reliable performance. It only needs several external resistors and capacitors to realize pulse generation and conversion circuits such as multivibrator, monostable trigger and Schmitt trigger. It is also widely used as a timer in instrumentation, household appliances, electronic measurement and automatic control.
The functions of the 555 timer pins are as follows:
Pin 1: External power supply negative terminal VSS or ground, under normal ground.
Pin 2: low trigger terminal TR.
Pin 3: output Vo
Pin 4: It is the direct clearing end. When this terminal is connected to a low level, the time base circuit does not work. At this time, regardless of the level of TR and TH, the time base circuit output is “0”, and the terminal should be connected to a high level when not in use.
Pin 5: VC is the control voltage terminal. If the external voltage is connected to this terminal, the reference voltage of the two internal comparators can be changed. When the terminal is not used, the terminal should be connected to a 0.01μF capacitor to ground to prevent interference.
Pin 6: high trigger terminal TH.
Pin 7: discharge end. This terminal is connected to the collector of the discharge tube and is used as a discharge of the capacitor when the timer is used.
Pin 8: External power supply VCC, the range of the bipolar time base circuit VCC is 4.5 ~ 16V, and the range of the CMOS type time base circuit VCC is 3 ~ 18V. Generally use 5V.
When the Pin 1 is grounded, the Pin 5 is not connected to the external voltage, and the two comparators A1 and A2 have the reference voltages respectively.
Input parameters:R1 (ohm); R2(ohm); C(F)
Output parameters:Duty cycle(%), High time(s),Low time(s), frequency(Hz)
TH (High time s) = 0.693 * (R1 + R2) * C
TL (Low time s) = 0.693 * R2 * C
Frequency(Output frequency) = 1.44 / ((R1 + R2 + R2) * C)
DCP(Duty cycle%) = (T(h) / (T(h) + T(l))) * 100