The 555 timer IC is often used to provide flexible astable oscillators and monostable timers. The IC is also capable of operating as a bistable latch which is used for the following example circuits.
The purpose of this article is to show the 555 timer IC used in its bistable mode as a hysteretic controller. The Threshold input is used to latch the output low while the Trigger input is used to latch the output high. Two example circuits are shown: one is a Low Voltage Disconnect (LVD), while the other is a Battery Charge Controller.
The timer contains two comparators referenced to 1/3 and 2/3 of its supply voltage which, with a regulated supply rail of 5V, sets the Threshold voltage to 3.33V and the Trigger voltage to 1.67V. These two fixed voltages, along with the resistor dividers R1/R2 and R3/R4 (as labelled in the following schematics) allow us to set the Threshold and Trigger points to be:
Setting the Threshold voltage (i.e. the input voltage above which the '555 output latches low):
Setting the Trigger voltage (i.e. the input voltage below which the '555 output latches high):
The LVD monitors the battery voltage and disconnects the load when the battery is sufficiently exhausted. It will automatically reconnect the load if the battery voltage should rise to a sufficient level (i.e. due to the battery being charged). With the values shown in the circuit, the Turn-On Voltage (VThreshold) is above ~12.6V while the Turn-Off Voltage (VTrigger) is below ~10.9V. It might be used to disconnect a refrigerator from the car battery when caravanning, for example, to stop the battery dying.
The Battery Charge Controller monitors the battery voltage and disconnects the charger if the battery voltage becomes too high to prevent overcharging. It will automatically reconnect the charger when the battery voltage falls below a set level (e.g. due to the battery being used). With the values shown in the circuit, the Turn-On Voltage (VTrigger) is below ~12.5V while the Turn-Off Voltage (VThreshold) is above ~14.4V. It could be used as a solar charge controller, for example.
In both example circuits the relay could be replaced with a P-channel MOSFET to reduce static power consumption if that is considered an issue. Also, the capacitor on the 5V rail would need to be increased if the charger voltage is not adequately filtered. Also, the value of the resistors in the resistor dividers can be scaled up (e.g. by 10x) to reduce the amount of current they draw. A potentiometer could be used in place of the resistor(s) to provide readily adjustable levels.
555 Timer as a Hysteretic Controller
The 555 timer IC is often used to provide flexible astable oscillators and monostable timers. The I