Clock & Timer Circuit DiagramsLCD-LED DisplayLights and Display Board Circuits

Backlight Delay Schematic Circuit Diagram

Many devices come equipped with a liquid crystal display (LCD), a feature that incorporates useful backlighting, enhancing the readability of displayed messages. However, in cases where continuous display reading is unnecessary, the backlight doesn’t need to remain illuminated all the time. Often, just a few seconds of backlighting is sufficient for reading purposes. This approach conserves power and extends the lifespan of the backlight.

Devices with LCDs also contain a processor, allowing for direct backlight control through software functions within the processor. However, there are situations where implementing such functions within the microcontroller is challenging. This could be due to all the controller’s pins being already utilized, or because the necessary source codes or tools required for modifying the software are unavailable.

Backlight Delay Schematic Circuit Diagram

This circuit is specifically designed to address such scenarios. In devices utilizing an LCD, there is usually at least one button that, upon pressing, pulls one of the microcontroller inputs down to 0 V. If such a button is absent, it can be easily added. The signal from this button serves as the control for the backlight. When the button is pressed, the backlight activates and turns off a few seconds later due to the integrated timer. Multiple buttons can be employed to trigger the timer through an OR gate arrangement.

Building this timer requires only a few components. The OR gate includes a pull-up resistor (R1+R2) and diodes corresponding to the number of buttons. These diodes allow transistor T1 to conduct while the button is pressed. Consequently, capacitor C1 charges rapidly, activating MOSFET T2, and illuminating the backlight.

The low value of resistor R3 ensures swift charging of capacitor C1, enabling even a brief press of a button to activate the timer. Upon button release, T1 deactivates, and C1 discharges gradually through R4 since T2 has a high input impedance. As T2’s gate voltage decreases, it turns off, and the backlight extinguishes. The duration the backlight remains lit after button release is approximately calculated as R4 (Ω) × C1 (F) seconds.

This versatile circuit is not limited to LED applications and can be adapted for various purposes, such as switching a relay. The choice of resistor R5 depends on the load being switched. For an LED operating on a 5 V supply, a value around 300 Ω is suitable.

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