Monostable Circuits

The 555 is a highly stable device for generating accurate time delays or oscillation. Aditional terminals are provided for triggering or resetting if desired. In the time delay (monostable) modeof operation the time is precisely controlled by one extrernal resistor and one capacitor. For stable operation as an oscillator, the free running frequency and the duty cycle are both accurately controlled with two external resistors and one capacitor.

This circuit was designed to detect when a call is incoming in a cellular phone (even when the calling tone of the device is switched-off) by means of a flashing LED. The device must be placed a few centimeters from the cellular phone, so its sensor coil L1 can detect the field emitted by the phone receiver during an incoming call. The signal detected by the sensor coil is amplified by transistor Q1 and drives the monostable input pin of IC1. The IC's output voltage is doubled by C2 & D2 in order to drive the high-efficiency ultra-bright LED at a suitable peak-voltage.

The circuit in Figure 1 satisfies all these criteria. The circuit delivers a unipolar (adjustable from 0 to 12V) pulse with adjustable frequency and pulse width. The first half of a dual, retriggerable monostable multivibrator, IC1A, generates the frequency of the pulse train. The 100-kΩ potentiometer, R1, along with R2 and C1, sets the adjustable frequency. R3, R4, and C2 set the adjustable pulse width in the second section of the multivibrator, IC1B.

An [...] idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty cycle will be proportional to capacitance. The ON time for the monostable is about 1.1RC, so component values that should work would be a 50 Hz clock, say a 1 Hz low-pass filter on the output, and R = 9.09K, 1%. That combination will give an output of one volt per microfarad. Switch R in decades for smaller capacitors. Trim R for calibration.

The µC-based, digitally programmable, monostable multivibrator in Figure 1 has more accurate timing than a conventional RC-based device because a 20-MHz crystal controls the µC. The PIC16C54 is a low-cost, high-speed, 18-pin µC. At a full speed of 20-MHz and using the accompanying program, this µC can generate programmable pulses of 10 µsec to 100 sec per step.

By combining the responses of an Analog Devices (www.analog.com) AD590 temperature sensor and a Humirel (www.humirel.com) HS1101 humidity sensor, you can generate a single TTL-level signal containing information from both sensors (Figure 1). This design uses a 74HC123 monostable multivibrator, IC1, to form a free-running oscillator.

By combining the responses of an Analog Devices (www.analog.com) AD590 temperature sensor and a Humirel (www.humirel.com) HS1101 humidity sensor, you can generate a single TTL-level signal containing information from both sensors (Figure 1). This design uses a 74HC123 monostable multivibrator, IC1, to form a free-running oscillator.

This circuit was designed to detect when a call is incoming in a cellular phone (even when the calling tone of the device is switched-off) by means of a flashing LED. The device must be placed a few centimeters from the cellular phone, so its sensor coil L1 can detect the field emitted by the phone receiver during an incoming call. The signal detected by the sensor coil is amplified by transistor Q1 and drives the monostable input pin of IC1. The IC's output voltage is doubled by C2 & D2 in order to drive the high-efficiency ultra-bright LED at a suitable peak-voltage. # Stand-by current drawing is less than 200µA, therefore a power on/off switch is unnecessary. Sensitivity of this circuit depends on the sensor coil type. L1 can be made by winding 130 to 150 turns of 0.2 mm. enameled wire on a 5 cm. diameter former (e.g. a can).

The Circuit Diagram is shown in Figure 1. It consists of a 4047 low-power monostable/astable multivibrator, IC1, used in the astable mode to provide the timing pulses to control the flash rate of the LEDs. To accomplish the astable mode, pins 4, 5, 6, and 14 are connected to +12VDC and pins 7, 8, 9, and 12 are connected to ground. Pins 1 and 3 are connected to C2 and pins 2 and 3 are connected to potentiometer R9. A fixed value resistor can be used in place of the potentiometer R9, if the flash rate does not need to be adjusted. These three pins make up the R-C timing circuit. The output pulses from the 4047 are taken from pins 10, 11, and 13. Pin 10 is the Q output and pin 11 is the Q-not output. These two pins are onnected to R6 and R7 respectively.

The Circuit Diagram is shown in Figure 1. It consists of a 4047 low-power monostable/astable multivibrator, IC1, used in the astable mode to provide the timing pulses to control the flash rate of the LEDs. To accomplish the astable mode, pins 4, 5, 6, and 14 are connected to +12VDC and pins 7, 8, 9, and 12 are connected to ground. Pins 1 and 3 are connected to C2 and pins 2 and 3 are connected to potentiometer R9. A fixed value resistor can be used in place of the potentiometer R9, if the flash rate does not need to be adjusted. These three pins make up the R-C timing circuit. The output pulses from the 4047 are taken from pins 10, 11, and 13. Pin 10 is the Q output and pin 11 is the Q-not output. These two pins are onnected to R6 and R7 respectively.

A zero-voltage-switching (ZVS) controller usually integrates a one-shot circuit, embodied in a VCO system. An error amplifier monitors the output voltage and adjusts the VCO's off-time to keep the output value at a constant level. Each on-time period commences as soon as the primary voltage drops to zero, thus eliminating on/off commutation losses associated with the switching element. The controller also incorporates other convenient features, such as MOSFET drivers, a voltage reference, and overvoltage and undervoltage lockouts. In low-cost circuits, such a complex architecture can lead to a prohibitive cost, especially if you don't need the cited features (in open-loop systems, for instance). Figure 1 shows an 8W, ZVS fluorescent-lamp converter made from two low-cost ICs, a CD4538 and an LM393.

The Control System uses an East and a West Cadmium Sulfide photoresistor (Sensors)to vary resistance to pin 2 of the corresponding 555 timer chip (East/West. The timer chips are wired for monostable "one shot" operation equal to approximately 1/10 of a second total output from pin 3 to the appropriate relay. The relays default to a positive voltage on either side of the motor (motor stopped). Light activates the appropriate timer/relay pair to momentarily ground one motor lead for motion.

The easy way to clamp a signal to a given value is to use two zener diodes, connected back-to-back. This method has several disadvantages. The accuracy of the clamping depends on the tolerance of the zener diodes, and the clamping is not adjustable, except by changing diodes. The circuit in Figure 1 is a bipolar clamper with a range of ±1 to ±10V, with the clamping level a function of the input VCLAMP.

Replace the timing resistor on a 74LS123 one-shot with a Howland current pump, drive the pump with a rail-to-rail voltage-output DAC, and you have a programmable one-shot with some unique features: single-supply operation, no clock required, a 25-to-1 pulse with adjustment range, and an "infinite" pulse-width capability. Figure 1 shows four such programmable one-shots having four overlapping ranges: 10 to 250 µsec, 100 to 2500 µsec, 1 to 25 µsec, and 10 to 250 msec. The following discussion refers to the top one-shot in Figure 1.

Resonant power supplies are popular because of high efficiency, low noise, and compactness. You can implement a resonant buck or boost converter using a single switch. The regulation of the output in such a converter derives from using a constant on or off time and a variable frequency.

Figure 3 shows a typical application circuit for the one-shot multivibrators. You can use IsSpice4 or PSpice to simulate this sample/hold circuit. Figure 4 shows the waveforms associated with the circuit in Figure 3. A PWM signal (top waveform) generates a kind of arbitrary staircase signal.

The schematic for a monostable multivibrator is shown in figure 3-11. Like the astable multivibrator, one transistor conducts and the other cuts off when the circuit is energized.

Figure 1 shows how to digitally program the on-time of a one-shot multivibrator circuit. More and more, the Internet is playing a role in control operations in industrial and R&D endeavors and in household appliances. One-shot circuits are popular choices for the on/off control circuitry.

You use a frequency discriminator to compare one signal frequency with another one. A functional feature, retriggering, of a monostable, one-shot 74xx123 multivibrator can yield frequency discrimination. Figure 1 shows a frequency discriminator that determines the relation of input-pulse frequency to a reference frequency. The external components, R1 and C1, set the reference frequency.

The circuit in Figure 1a remembers the width of an input pulse at the in terminal. After the input pulse returns to zero, a trigger input causes an output pulse to occur at the out terminal. This output pulse is proportional to the width of the input pulse. R2 and R3 set this pulse-width ratio. For the values shown, the output pulse is half the width of the input pulse, but you can set the output-pulse width to any fraction or multiple by selecting the ratio of R2/R3. The circuit's operation depends on this resistor ratio but is independent of power-supply variation, temperature drift, and the tolerances of R1 and C1.