A project I've dabbled with on and off deals with using the degree of 'openness' of ones hand to vary the light output of an LED, to create a simple to control, hand-mounted flashlight.
Video: A proof of concept test using a high-brightness (< 1W) LED.
I worked extensively within NI MultiSim to design and test the circuit before purchasing the components. The proof of concept circuit used a standard rotary potentiometer as a voltage divider to drive the gate of a transistor, allowing current to flow and powering the LED. Later prototype circuits will utilize a flexible stretch sensor, like this one.
While most potentiometers have a minimum value near zero ohms, the stretch potentiometer's minimum value is around 10k ohm for a 12" segment (with a max around 40k ohm) - this made it highly impractical to develop a simple resistor-divider based input stage to the driving transistor.
After going through many test circuits and iterations of component values and types, I determined that the simple divider wouldn't allow the full brightness range in the high power (> 1W) LED without major sacrifices: either the LED wouldn't reach an 'off' value, it wouldn't fall quite short of max brightness, or it would waste a lot of power as heat from the limiting resistor.
The first full test circuit uses 4 major circuit subsystems: the 555 timer, a voltage divider using the stretch potentiometer, a comparator chip, and the MOSFET LED driver. The voltage from the stretch pot system is compared to a sawtooth signal resulting from the 555's charge-discharge cycle, and the resulting PWM signal is sent to an n-channel enhancement mode MOSFET which drives the LED. By tuning the resistor values in the voltage divider, any length of stretch potentiometer can be accommodated.
Parts have been ordered for the 555 PWM circuit, and assembly and test can begin soon. I have also ordered parts for a potentially improved version utilizing an LED driver IC, the LM3405.
A Change of Plan
One of the design requirements is that the light should go from off to max within the standard range of motion of opening the hand. This was simple with a basic rotary potentiometer, but became complicated when moving to a circuit using the stretch potentiometer.While most potentiometers have a minimum value near zero ohms, the stretch potentiometer's minimum value is around 10k ohm for a 12" segment (with a max around 40k ohm) - this made it highly impractical to develop a simple resistor-divider based input stage to the driving transistor.
After going through many test circuits and iterations of component values and types, I determined that the simple divider wouldn't allow the full brightness range in the high power (> 1W) LED without major sacrifices: either the LED wouldn't reach an 'off' value, it wouldn't fall quite short of max brightness, or it would waste a lot of power as heat from the limiting resistor.
Simple PWM
I decided to switch tactics, and focus on a PWM based system, utilizing a 555 timer oscillating in astable mode. This circuit worked very well in simulation and solved a lot of the problems I was having.The first full test circuit uses 4 major circuit subsystems: the 555 timer, a voltage divider using the stretch potentiometer, a comparator chip, and the MOSFET LED driver. The voltage from the stretch pot system is compared to a sawtooth signal resulting from the 555's charge-discharge cycle, and the resulting PWM signal is sent to an n-channel enhancement mode MOSFET which drives the LED. By tuning the resistor values in the voltage divider, any length of stretch potentiometer can be accommodated.
Image: The MultiSim schematic for the first full test circuit - 555 PWM.
Parts have been ordered for the 555 PWM circuit, and assembly and test can begin soon. I have also ordered parts for a potentially improved version utilizing an LED driver IC, the LM3405.
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