Most computer cases typically feature a single LED to indicate hard disk activity, linked to the motherboard via a two-pin connector. However, this LED is compatible only with IDE drives. If a SCSI disk controller is installed, its activity remains unnoticed. This issue is resolved by a simple circuit using a multicolour LED. The activity LED for the IDE interface is typically controlled by a connected device through open-collector stages.
It lights up when either of the two possible IDE drives is active. The shared series resistor controls the current and offers short-circuit protection. Even in the event of a short due to faulty wiring, the current remains at a safe level. A potential solution could involve having both IDE and SCSI disks drive a shared dual LED. However, current flows from the positive supply line through a series resistor, the LED, and a transistor to the ground, making this approach unfeasible.
Creating a Common Anode Connection for Dual LEDs
In this scenario, the challenge lies in establishing a common anode connection for dual LEDs, a configuration not readily available in existing devices. Multicolour LEDs typically feature a shared cathode lead, making direct connection impossible. However, this obstacle can be overcome with a simple additional circuit. By routing the drive signals from two controllers through optocouplers, which mimic the behavior of the original LED, the problem of different interfaces is resolved. Optocouplers, in tandem with a Darlington transistor composed of a phototransistor and a BC547, efficiently drive the multicolour LED. The infrared LED’s slightly lower voltage drop, compensated by a higher current, ensures minimal risk of overload.
Adjustable Series Resistors for Optimal LED Operation
To ensure proper functioning, a 10-kΩ resistor, with a non-critical value, serves as a safeguard for cutting off the driver transistor. The base of the phototransistor within the CNY17 remains open. To determine the series resistors for individual LED elements, a standard formula guides their dimensioning. Slight adjustments to these values may be necessary based on relative brightness levels. Moreover, modifications to the series resistor values allow the circuit to operate from the +12-V power supply line if needed.
Expanding Possibilities with Three-Colour LEDs
For those seeking expanded options, a third optocoupler stage can be incorporated, enabling the control of three-colour LEDs (red, green, and blue). The circuit board’s compact design ensures easy installation, with components fitting seamlessly in a few minutes. To prevent displacement due to vibration, a drop of hot-melt glue secures the circuit board. Supply voltage access is facilitated through a standard small drive connector, simplifying the acquisition of the necessary plug. Alternatively, ordinary solder pins can also serve this purpose.