Adjustable Light Glove, Part 2 - Prototype Circuit v2.2

LinkBack => Part 1 - Concept, and Proof-of-Concept <Blogspot>

Previously, I discussed the beginning hurdles in my attempt to to create an intuitive, hand-mounted flashlight. Since then, I've worked to refine the circuit for the first prototype which uses the stretch sensor.

Image: Prototype circuit v2.2 - This uses a simplified charging solution for the 

555, a voltage divider to provide 2.4V on the Control pin, and

the correct values for the stretch sensor network.

 

Video: Prototype circuit v2.2 - This uses a 555-based PWM and a 0.75" section 

of stretch sensor to control the light output of a Luxeon 1W LED.

Below I'll discuss some of the challenges and decisions I've made on this leg of the process. The next steps are to test the LM3405 LED Constant Current Buck Regulator to see if it will meet my needs more thoroughly than this test-bed style setup can. After that, it's time to design, etch, and populate a PCB for the first full device prototype.

Trials and Tribulations

A major point of concern was ensuring that the circuit was built so that it could be modified easily to accommodate various lengths of stretch sensor (and these changes could be determined easily). While working with the circuit, the main problems I came across were:

1. LED Light 'strobing' 
2. Tuning the circuit to allow the maximum duty cycle range, with emphasis placed on the low end (ie, it was more important that the circuit reach 0% duty cycle than 100%)

 Strobing

The first problem, strobing, was due to the frequency of the 555 system being too low. I couldn't find formula detailing the frequency expected when using the transistor charging circuit, and I didn't want to base the solution on trial and error. Some research led me to decide that the transistor portion was unnecessary (for this purpose, I don't need a perfect ramp - the sawtooth capacitor charge-discharge pattern is sufficient). Replacing it with the basic astable 555 circuit (two resistors and a capacitor) simplified the frequency calculation. The formula used was: Freq = 1.44 / ((R1+2R2)*C)

Maths were called for, so I plugged in some various combinations into an Excel doc I made for the circuit. I decided on a combination to give me a frequency of 444 Hz - changes made later, specifically using the 555's Control pin, ended up raising this to 750 Hz. Updated the proto board, and no more flicker.

Duty Cycle

Using the aforementioned Excel spreadsheet, I tried various experimental combinations of resistor values for the stretch sensor sub-circuit. This data showed me that with my length of stretch sensor, while I can control the actual Vout,max and Vout,min values, the difference in their values is more or less fixed at 1.2V (in the range of values I'm considering, 360ohm to 1600ohm). The two options that jumped out at me were to work with the 555 sub-circuit, or to place something between the stretch sensor network and the comparator to increase the network's voltage range. I opted to work with the 555 sub-circuit to make that 1.2V gap work.Since the current gap of 1.67V is too large, it needed to be changed. Since the 555 needs at least 4.5V to operate, I couldn't lower its input voltage low enough to achieve the 1.2V spacing, so I opted to make use of its Control pin. By applying a voltage to CON, the max voltage of the sawtooth signal is set to the voltage at CON, while the min sawtooth voltage is 1/2 the CON voltage. By choosing a value of 2.4V at CON, I gain the 1.2V gap I'm looking for. For simplicities sake (and easy testing), I'm using a simple voltage divider to achieve the 2.4V reference, as I had the parts already.

I used the spreadsheet to find a value of resistor for the stretch network to give me a Vout range of 1.26V to 2.42V (compared to the 555's sawtooth of 1.2V to 2.4V). Since the stretch Vmax exceeds the sawtooth Vmax, the circuit achieves the goal of 0% duty cycle (with a max around 92%).