LM555
A wonderfully creative circuit. Basically it is an SR latch with a voltage comparator on the inputs. Between the two voltage comparator inputs is a voltage divider between +V and ground with one tap at 2/3 +V and another tap at 1/3 +V. Taking a look at the pinout with this in mind we have …
1 – Ground
2 – Trigger input. An input to a voltage comparator. When this input goes lower than 1/3 of +V the latch sets and the output goes high. The Discharge output is released.
3 – Output. Q output of the latch. On the LM555 this can sink or source 200 mA.
4 – Reset input. A low clears the latch.
5 – Compensation. “Control input”. It is tied to the higher tap in the voltage divider. Normally it just has a small cap to ground. Creatively it can be used as an input to modify the timing cycle.
6 – Threshold input. This goes to the other voltage comparator input. When the voltage on this pin goes higher than 2/3 +V the latch clears and the Discharge pin goes low.
7 – Discharge output. An open collector output that goes low when the latch clears.
8 – The +V line. Specs are guaranteed between 4.5 V and 16 V but this doesn’t mean it can’t operate a bit outside these limits. Typical current drain is about 3 mA at 4.5 V to 15 mA at 15 V.
As a single shot (monostable multivibrator) …
Threshold and Discharge are tied together. A resistor is tied from here to +V and a capacitor to ground. When Trigger goes lower than 1/3 +V the latch sets, Discharge is released, the output goes high and the capacitor starts charging. When the voltage on the capacitor reaches 2/3 +V the latch clears, the output goes low and the Discharge output goes low discharging the capacitor.
As an oscillator (astable multivibrator) …
One resistor is tied to Discharge and +V. Threshold is tied to Trigger and a resistor is tied between these two and Discharge. A capacitor is tied between Threshold / Trigger and ground. On initial power up the capacitor is discharged, Trigger is less than 1/3 +CV so the latch sets and the output goes high. Discharge is released. The capacitor is charged through both resistors. Eventually the capacitor charges to 2/3 +V. Threshold is met. The latch clears. The output goes low and Discharge goes low, discharging the capacitor through the lower resistor. When the voltage on the capacitor goes below 1/3 +V the latch sets again and the process repeats itself.
Design 491, Creative use of an LM555.
In theory this would improve the efficiency of a power supply. Normally the load current is drained off of the output capacitor. Proper design has it that this capacitor can hold a charge long enough to last between pulses from the AC line. Here we monitor the charge on the capacitor. When the cap drains down to a certain point the LM555 latch sets and the voltage regulator turns on charging the capacitor. When the cap is sufficiently charged the LM555 latch clears and the voltage regulator turns off, saving power. On light loads the regulator is off most of the time saving power (in theory).
A wonderfully creative circuit. Basically it is an SR latch with a voltage comparator on the inputs. Between the two voltage comparator inputs is a voltage divider between +V and ground with one tap at 2/3 +V and another tap at 1/3 +V. Taking a look at the pinout with this in mind we have …
1 – Ground
2 – Trigger input. An input to a voltage comparator. When this input goes lower than 1/3 of +V the latch sets and the output goes high. The Discharge output is released.
3 – Output. Q output of the latch. On the LM555 this can sink or source 200 mA.
4 – Reset input. A low clears the latch.
5 – Compensation. “Control input”. It is tied to the higher tap in the voltage divider. Normally it just has a small cap to ground. Creatively it can be used as an input to modify the timing cycle.
6 – Threshold input. This goes to the other voltage comparator input. When the voltage on this pin goes higher than 2/3 +V the latch clears and the Discharge pin goes low.
7 – Discharge output. An open collector output that goes low when the latch clears.
8 – The +V line. Specs are guaranteed between 4.5 V and 16 V but this doesn’t mean it can’t operate a bit outside these limits. Typical current drain is about 3 mA at 4.5 V to 15 mA at 15 V.
As a single shot (monostable multivibrator) …
Threshold and Discharge are tied together. A resistor is tied from here to +V and a capacitor to ground. When Trigger goes lower than 1/3 +V the latch sets, Discharge is released, the output goes high and the capacitor starts charging. When the voltage on the capacitor reaches 2/3 +V the latch clears, the output goes low and the Discharge output goes low discharging the capacitor.
As an oscillator (astable multivibrator) …
One resistor is tied to Discharge and +V. Threshold is tied to Trigger and a resistor is tied between these two and Discharge. A capacitor is tied between Threshold / Trigger and ground. On initial power up the capacitor is discharged, Trigger is less than 1/3 +CV so the latch sets and the output goes high. Discharge is released. The capacitor is charged through both resistors. Eventually the capacitor charges to 2/3 +V. Threshold is met. The latch clears. The output goes low and Discharge goes low, discharging the capacitor through the lower resistor. When the voltage on the capacitor goes below 1/3 +V the latch sets again and the process repeats itself.
Design 491, Creative use of an LM555.
In theory this would improve the efficiency of a power supply. Normally the load current is drained off of the output capacitor. Proper design has it that this capacitor can hold a charge long enough to last between pulses from the AC line. Here we monitor the charge on the capacitor. When the cap drains down to a certain point the LM555 latch sets and the voltage regulator turns on charging the capacitor. When the cap is sufficiently charged the LM555 latch clears and the voltage regulator turns off, saving power. On light loads the regulator is off most of the time saving power (in theory).
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