Showing posts with label timer. Show all posts
Showing posts with label timer. Show all posts
Wednesday, October 1, 2014
Simple UJT Repeat Timer Circuit
The precision time delay circuit shown will provide accurate and repeatable time delays adjustable from a few milliseconds to a minute or two.
This is a very flexible circuit in which the operating currents and voltage depends only on the choice of SCR. The timing sequence may be initiated either by applying power to the circuit or by opening a shorting switch wired across C1. Timing capacitor C1 is charged via R1 and R2 until the voltage across C1 reaches the peak point voltage of the UJT O1. When this occurs, Q1 fires, generating a pulse across R4, triggering the SCR, and applying power to the load. Holding current for the SCR is provided via R5 and D1. The circuit is ireset by momentarily removing the supply voltage. If the circuit is to be used in an application where both rapid cycling and accurate, repeatable timing is required - some provision must be made to ensure that C1 is discharged to zero before each-timing sequence. This can most easily be done by interconnecting a pair of switch contacts with the reset system so as to momentarily shortout C1 whenever the circuit is reset. Temperature compensation for this circuit is provided by _R3._|increasing the value of this resistor causes the circuit to have a positive temperature coefficient. It is possible to obtain zero coefficient over a small range of ambient temperatures by optimizing » R3.
Simple UJT Repeat Timer Circuit
This is a very flexible circuit in which the operating currents and voltage depends only on the choice of SCR. The timing sequence may be initiated either by applying power to the circuit or by opening a shorting switch wired across C1. Timing capacitor C1 is charged via R1 and R2 until the voltage across C1 reaches the peak point voltage of the UJT O1. When this occurs, Q1 fires, generating a pulse across R4, triggering the SCR, and applying power to the load. Holding current for the SCR is provided via R5 and D1. The circuit is ireset by momentarily removing the supply voltage. If the circuit is to be used in an application where both rapid cycling and accurate, repeatable timing is required - some provision must be made to ensure that C1 is discharged to zero before each-timing sequence. This can most easily be done by interconnecting a pair of switch contacts with the reset system so as to momentarily shortout C1 whenever the circuit is reset. Temperature compensation for this circuit is provided by _R3._|increasing the value of this resistor causes the circuit to have a positive temperature coefficient. It is possible to obtain zero coefficient over a small range of ambient temperatures by optimizing » R3.
Simple UJT Repeat Timer Circuit
Thursday, September 25, 2014
Snooze Delay Timer Circuit
When the Set switch is depressed the large electrolytic capacitor is charged via the limiting resistor (lk). This charge causes the BC109 to conduct which supplies enough base current to switch on the 2N1711 space and operate the relay.
The relay contacts are wired in parallel with the mains switch so that if the mains switch is now turned off, the equipment will continue.The supply·voltage is taken from the equipment in which the unit is fitted and will determine the choice of` relay. The maximum delay being 1.75 hours.
Snooze Delay Timer Circuit
The relay contacts are wired in parallel with the mains switch so that if the mains switch is now turned off, the equipment will continue.The supply·voltage is taken from the equipment in which the unit is fitted and will determine the choice of` relay. The maximum delay being 1.75 hours.
Snooze Delay Timer Circuit
Wednesday, September 24, 2014
Simple Mutifunction Timer Circuit
A timer is almost impossible without IC 555 or counter ICs such as CD4029 or CD4060. The circuit presented here uses an inverter instead for doing thc samc job.
As shown in the proposed multifunction timer circuit, gatc N1 along with C1, R1, VR1 and VR2 forms 21 monostable multivibrator. When S1 and S2, which are ganged, are in position 2, input at gate N1 is held low and hence the output of gate N1 becomes high. This high output charges C1 via A timer is almost impossible without IC 555 or counter ICs such as CD4029 or CD4060. The circuit pre- sented here uses an inverter instead for doing thc same job. As shown in the circuit, gatc N1 along with C1, R1, VR1 and VR2 forms 21 monostable multivibrator. When S1 and S2, which are ganged, are in position 2, input at gate N1 is held low and hence the output of gate N1 becomes high. This high output charges C1 via V series combination of R1, VR1 andVR2. At the same time output of gate N2, which is low, is given to gate N6 and the output of gate N6 becomes high.
The high output triggers the triac and the power is fed to the load. After a time period 0fT = 0.7 x C1 (R1 + VR1 + VR2), the output of gate N6 becomes low and the triac stops conducting. Thus this part of the circuit operates in monoshot mode.
When Sl and S2 are at position 1, the input of gate N6 is connected to the astable multivibrator built around gates N3 and N4.
Thus, the load is interrupted at regular intervals. Here VR3 is ganged with VR5, and VR4 is ganged with VR6 to generate a square wave. Duty cycle of the output can be varied by just removing the gang arrangement (because the at charging and discharging time will be different). Here gates N2 and N5 are as acting as buffers.
When Sl and S2 are in position 3, the input of gate N6 is grounded and so the output of N6 remains high as long as s the switches are in this position. Hence, the triac remains on and the current s flows continuously through the load. Finally, when Sl and S2 are in position 4, the input of N6 is connected to Vcc as a result of which the output re- mains at logic 0. The triac does not conduct and thus there is no current in the load.
330k pots arc calibrated for 0 to 60 seconds and 10M pots arc calibrated for 0 to 30 minutes. Thus, one can obtain any desired timing from 0 to 30 minutes. Use of tantalum capacitors will surely enhance the accuracy of the cir- cuit but at the same time the cost will also rise. Separate on/off switch is not required because stand-by mode consumption is only a few microamperes. In full load condition, the circuit consumes around 5 to 6 mA current ensuring a long life for the battery.
This kind of time switch can be used r as a timer for washing machines, heat- ers and for process control involving short timing etc. The circuit can be easily wired onla general-purpose PCB. It costs around Rs 50, without power supply.
As shown in the proposed multifunction timer circuit, gatc N1 along with C1, R1, VR1 and VR2 forms 21 monostable multivibrator. When S1 and S2, which are ganged, are in position 2, input at gate N1 is held low and hence the output of gate N1 becomes high. This high output charges C1 via A timer is almost impossible without IC 555 or counter ICs such as CD4029 or CD4060. The circuit pre- sented here uses an inverter instead for doing thc same job. As shown in the circuit, gatc N1 along with C1, R1, VR1 and VR2 forms 21 monostable multivibrator. When S1 and S2, which are ganged, are in position 2, input at gate N1 is held low and hence the output of gate N1 becomes high. This high output charges C1 via V series combination of R1, VR1 andVR2. At the same time output of gate N2, which is low, is given to gate N6 and the output of gate N6 becomes high.
The high output triggers the triac and the power is fed to the load. After a time period 0fT = 0.7 x C1 (R1 + VR1 + VR2), the output of gate N6 becomes low and the triac stops conducting. Thus this part of the circuit operates in monoshot mode.
When Sl and S2 are at position 1, the input of gate N6 is connected to the astable multivibrator built around gates N3 and N4.
Thus, the load is interrupted at regular intervals. Here VR3 is ganged with VR5, and VR4 is ganged with VR6 to generate a square wave. Duty cycle of the output can be varied by just removing the gang arrangement (because the at charging and discharging time will be different). Here gates N2 and N5 are as acting as buffers.
When Sl and S2 are in position 3, the input of gate N6 is grounded and so the output of N6 remains high as long as s the switches are in this position. Hence, the triac remains on and the current s flows continuously through the load. Finally, when Sl and S2 are in position 4, the input of N6 is connected to Vcc as a result of which the output re- mains at logic 0. The triac does not conduct and thus there is no current in the load.
330k pots arc calibrated for 0 to 60 seconds and 10M pots arc calibrated for 0 to 30 minutes. Thus, one can obtain any desired timing from 0 to 30 minutes. Use of tantalum capacitors will surely enhance the accuracy of the cir- cuit but at the same time the cost will also rise. Separate on/off switch is not required because stand-by mode consumption is only a few microamperes. In full load condition, the circuit consumes around 5 to 6 mA current ensuring a long life for the battery.
This kind of time switch can be used r as a timer for washing machines, heat- ers and for process control involving short timing etc. The circuit can be easily wired onla general-purpose PCB. It costs around Rs 50, without power supply.
Saturday, September 20, 2014
High Low Voltage Cutout Without Timer
This inexpensive circuit can be connected to an air-conditioner/fridge or to any other sophisticated electrical appliance for its protection. Generally, costly voltage stabilizers are used with such appliances for maintaining constant AC voltage. However, due to fluctuations in AC mains supply, a regular ‘click’ sound in the relays is heard. The frequent energisation/de-energisation of the relays leads to electrical noise and shortening of the life of electrical appliances and the relay/stabilizer itself. The costly yet fault-prone stabiliser may be replaced by this inexpensive high-low cutout circuit with timer.
The circuit is so designed that relay RL1 gets energised when the mains voltage is above 270V. This causes resistor R8 to be inserted in series with the load and thereby dropping most of the voltage across it and limiting the current through the appliance to a very low value. If the input AC mains is less than 180 volts or so, the low-voltage cut-off circuit interrupts the supply to the electrical appliance due to energisation of relay RL2. After a preset time delay of one minute (adjustable), it automatically tries again. If the input AC mains supply is still low, the power to the appliance is again interrupted for another one minute, and so on, until the mains supply comes within limits (>180V AC).
Circuit diagram:
The AC mains supply is resumed to appliance only when it is above the lower limit. When the input AC mains increases beyond 270 volts, preset VR1 is adjusted such that transistor T1 conducts and relay RL1 energises and resistance R8 gets connected in series with the electrical appliance. This 10-kilo-ohm, 20W resistor produces a voltage drop of approximately 200V, with the fridge as load. The value and wattage of resistor R8 may be suitably chosen according to the electrical appliance to be used. It is practically observed that after continuous use, the value of resistor R8 changes with time, due to heating. So adjustment of preset VR1 is needed two to three times in the beginning.
But once it attains a constant value, no further adjustment is required. This is the only adjustment required in the beginning, which is done using a variac. Further, the base voltage of transistor T2 is adjusted with the help of preset VR2 so that it conducts up to the lower limit of the input supply and cuts off when the input supply is less than this limit (say, 180V). As a result, transistor T3 remains cut off (with its collector remaining high) until the mains supply falls below the lower limit, causing its collector voltage to fall. The collector of transistor T3 is connected to the trigger point (pin 2) of IC1. When the input is more than the lower limit, pin 2 of IC1 is nearly at +Vcc.
In this condition the output of IC1 is low, relay RL2 is de-energised and power is supplied to the appliance through the N/C terminals of relay RL2. If the mains supply is less than the lower limit, pin 2 of IC1 becomes momentarily low (nearly ground potential) and thus the output of IC1 changes state from ‘low’ to ‘high’, resulting in energisation of relay RL2. As a result, power to the load/appliance is cut off. Now, capacitor C2 starts charging through resistor R6 and preset VR3. When the capacitor charges to (2/3)Vcc, IC1 changes state from ‘high’ to ‘low’. The value of preset VR3 may be so adjusted that it takes about one minute (or as desired) to charge capacitor C1 to (2/3)Vcc.
Relay is now de-energised and the power is supplied to the appliance if the mains supply voltage has risen above the lower cut-off limit, otherwise the next cycle repeats automatically. One additional advantage of this circuit is that both relays are de-energised when the input AC mains voltage lies within the specified limit and the normal supply is extended to the appliance via the N/C contacts of both relays.
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The circuit is so designed that relay RL1 gets energised when the mains voltage is above 270V. This causes resistor R8 to be inserted in series with the load and thereby dropping most of the voltage across it and limiting the current through the appliance to a very low value. If the input AC mains is less than 180 volts or so, the low-voltage cut-off circuit interrupts the supply to the electrical appliance due to energisation of relay RL2. After a preset time delay of one minute (adjustable), it automatically tries again. If the input AC mains supply is still low, the power to the appliance is again interrupted for another one minute, and so on, until the mains supply comes within limits (>180V AC).
Circuit diagram:
The AC mains supply is resumed to appliance only when it is above the lower limit. When the input AC mains increases beyond 270 volts, preset VR1 is adjusted such that transistor T1 conducts and relay RL1 energises and resistance R8 gets connected in series with the electrical appliance. This 10-kilo-ohm, 20W resistor produces a voltage drop of approximately 200V, with the fridge as load. The value and wattage of resistor R8 may be suitably chosen according to the electrical appliance to be used. It is practically observed that after continuous use, the value of resistor R8 changes with time, due to heating. So adjustment of preset VR1 is needed two to three times in the beginning.
But once it attains a constant value, no further adjustment is required. This is the only adjustment required in the beginning, which is done using a variac. Further, the base voltage of transistor T2 is adjusted with the help of preset VR2 so that it conducts up to the lower limit of the input supply and cuts off when the input supply is less than this limit (say, 180V). As a result, transistor T3 remains cut off (with its collector remaining high) until the mains supply falls below the lower limit, causing its collector voltage to fall. The collector of transistor T3 is connected to the trigger point (pin 2) of IC1. When the input is more than the lower limit, pin 2 of IC1 is nearly at +Vcc.
In this condition the output of IC1 is low, relay RL2 is de-energised and power is supplied to the appliance through the N/C terminals of relay RL2. If the mains supply is less than the lower limit, pin 2 of IC1 becomes momentarily low (nearly ground potential) and thus the output of IC1 changes state from ‘low’ to ‘high’, resulting in energisation of relay RL2. As a result, power to the load/appliance is cut off. Now, capacitor C2 starts charging through resistor R6 and preset VR3. When the capacitor charges to (2/3)Vcc, IC1 changes state from ‘high’ to ‘low’. The value of preset VR3 may be so adjusted that it takes about one minute (or as desired) to charge capacitor C1 to (2/3)Vcc.
Relay is now de-energised and the power is supplied to the appliance if the mains supply voltage has risen above the lower cut-off limit, otherwise the next cycle repeats automatically. One additional advantage of this circuit is that both relays are de-energised when the input AC mains voltage lies within the specified limit and the normal supply is extended to the appliance via the N/C contacts of both relays.
Monday, September 1, 2014
Using 555 Timer Voltage Controlled Switch
In this schema the 555 timer is used in a novel way, as a voltage controlled switch.The old and omnipresent NE555 can be very good at something it was not meant for: driving relays or other loads up to 200 mA. The picture shows an example schema: if the input level rises over 2/3 of the supply voltage - it will turn on the relay, and the relay will stay on until the level at the input drops below one third of the supply voltage.
If the relay and D1 were connected between pin 3 and ground, the relay would be activated when the input voltage drops below one third, and deactivated when the input voltage goes over two thirds of the supply voltage. It is also a nice advantage that the input requires only about 1 uA, which is something bipolar transistors cant compete with. (This high impedance input must not be left open.) A large hysteresis makes the schema immune to noise. The output (pin 3) can only be either high or low (voltage-wise), and it changes its state almost instantenously, regardless of the input signal shape.
If the relay and D1 were connected between pin 3 and ground, the relay would be activated when the input voltage drops below one third, and deactivated when the input voltage goes over two thirds of the supply voltage. It is also a nice advantage that the input requires only about 1 uA, which is something bipolar transistors cant compete with. (This high impedance input must not be left open.) A large hysteresis makes the schema immune to noise. The output (pin 3) can only be either high or low (voltage-wise), and it changes its state almost instantenously, regardless of the input signal shape.
Voltage Controlled Switch Circuit Diagram
The voltage drop across the NE555s output stage (at 35-100 mA) is 0.3-2.0 V, depending on the way the relay is connected and the exact current it draws. D1 is absolutely vital to the safety of the integrated schema.
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