The tech net tonight will introduce the NE555 timer chip. We will discuss a monostable configuration that could be used for an identification timer. The one we will discuss will be about 6 minutes.
R1 – 1Mohm
C1 – 330uf
Time = 1.1 x R x C
Ground supply: this pin is the ground reference voltage (zero volts).
Trigger: when the voltage at this pin falls below 1⁄2 of CONT pin voltage (1⁄3 VCC except when CONT is driven by an external signal), the OUT pin goes high and a timing interval starts. As long as this pin continues to be kept at a low voltage, the OUT pin will remain high.
Output: this is a push-pull (P.P.) output that is driven to either a low state (ground supply at GND pin) or a high state (positive supply at VCC pin minus approximately 1.7 Volts). (Note: For CMOS timers, the high state is driven to VCC.) When bipolar timers are used in applications where the output drives a TTL input, a 100 to 1000 pF decoupling capacitor may need to be added to prevent double triggering.
Reset: a timing interval may be reset by driving this pin to GND, but the timing does not begin again until this pin rises above approximately 0.7 Volts. This pin overrides TRIG (trigger), which overrides THRES (threshold). In most applications this pin is not used, thus it should be connected to VCC to prevent electrical noise causing a reset.
Control (or Control Voltage): this pin provides access to the internal voltage divider (2⁄3 VCC by default). By applying a voltage to the CONT input one can alter the timing characteristics of the device. In most applications this pin is not used, thus a 10 nF decoupling capacitor (film or C0G) should be connected between this pin and GND to ensure electrical noise doesn't affect the internal voltage divider. This control pin input can be used to build an astable multivibrator with a frequency-modulated output.
Threshold: when the voltage at this pin is greater than the voltage at CONT pin (2⁄3 VCC except when CONT is driven by an external signal), then the timing (OUT high) interval ends.
Discharge: this is an open-collector (O.C.) output (CMOS timers are open-drain), which can be used to discharge a capacitor between intervals, in phase with output.
Positive supply: the guaranteed voltage range of bipolar timers is typically 4.5 to 15 Volts (some timers are spec'ed for up to 16 Volts or 18 Volts), though most will operate as low as 3 Volts. (Note: CMOS timers have a lower minimum voltage rating, which varies depending on the part number.) See the supply min and max columns in the derivatives table. For bipolar timers, a decoupling capacitor is required because of current surges during output switching.