If you've even included digital logic, analog chips, or a microcontroller into a project there's a good chance you've had to find a way to provide 5V DC to your circuit. There are few primary sources of 5V, so you can use a wall wart to convert AC power (which obviously limits where you can take your new gadget) or you can spend extra time building a regulator circuit to get multiple 1.5V batteries to the needed voltage. These solutions are required for some circuits, but for smaller gadgets, wouldn't it be nice to have an always-ready supply so you can go straight to working on other aspects of the project?
By adding a few electronic components to a widely available dynamo flashlight, you can power small devices for short periods without using up outlets or batteries. The improved dynamo is great for the workbench or showing off new projects just about anywhere.
This Instructable covers how to assemble and install a step-up DC-DC converter that turns the varying low voltage of the keychain dynamo's generator into a constant 5V. The step-up circuit charges a large capacitor which provides energy storage and some power even when the dynamo isn't turning.
By following the steps in this Instructable, you can accomplish all this without manufacturing a special circuit board or using hard-to-solder surface mount components. To get the electronic parts inside the keychain case requires some circuit origami, but after about an hour of tinkering you will have a neat device that can source up to 50 milliamps of current at a constant 5V DC while winding and milliwatts of power for minutes after!
Step 1: How it works
Current flowing into a motor creates a magnetic field in coils attached to the shaft, which turns in the presence of a magnetic field from fixed magnets. When a motor is run in reverse -- power is applied by turning the shaft -- a voltage is induced in the coil. Faraday's law says this voltage is proportional to the rate the magnetic field changes in the coil. Thus the faster the shaft is turned, the greater the voltage.
A series of gears are used within the keychain to get the generator spinning as fast as possible. When you crank the handle, it sets three compound spur gears in motion. One half of each compound gear has a small radius and the other half has a large radius. When the small radius is turned, the teeth at the edge of the larger radius change location at a proportionally faster rate. By cascading these compound gears, the cranking rate can be multiplied several times and the generator shaft can be turned much faster than a human could turn it.
The need for the step-up converter and storage capacitor
The keychain's gear ratio can generate a few volts with reasonable cranking, but the voltage is not high enough to reach 5V. This voltage also varies quickly based on the shaft rotation rate. To get a steady 5V output, a step-up converter is needed. The specific integrated circuit chosen -- the MAX756 -- can turn voltages as low as 0.7V into 5V and comes in a handy 8 pin package. The step-up circuit is based on the application circuit in the MAX756 datasheet.
Even though these dynamo keychain flashlights are advertised as needing no batteries, they appear to have three coin-sized batteries inside. The generator is soldered to this coin battery stack in a somewhat crude charging circuit. However, I do not think these batteries are meant to be rechargeable, and they tend to drain quickly after the initial discharge. This Instructable replaces this coin stack with a large capacitor that can be recharged more frequently and is more efficient.
See the schematic for the layout of the entire circuit. The specific components were chosen for easy hand soldering while being the smallest sizes that were still rated for the voltages in the circuit. Note: The MAX756 datasheet has C3 as a 150 uF capacitor. The 150 uF capacitors I found were much physically bigger than the 100 uF ones and wouldn't fit within the small keychain. Thus I replaced C3 with a 100 uF capacitor and it appears to work fine.
Step 2: Parts and Tools
The parts for the step-up circuit can be obtained from an electronics distributor such as Digikey.
U1 -- MAX756 3.3V/5V step-up DC-DC converter, 8-pin DIP package [Digikey# MAX756CPA+-ND]
C1 -- 0.33 F 5.5V capacitor, coin package [Digikey# 604-1024-ND]
C2,C3 -- 100 uF 6.3V aluminum electrolytic capacitor, mini radial [Digikey# P803-ND]
C4 -- 0.1 uF 25V ceramic general purpose capacitor, through-hole [Digikey# BC1148CT-ND]
L1 -- 22 uH RF choke, axial [Digikey# M8138CT-ND]
R1 -- 1k, 1/4W general purpose carbon film resistor, axial [Digikey# 1.0KQBK-ND]
D2 -- 1A 20V Schottky diode, axial [Digikey# 1N5817GOS-ND]
D3 -- If you are not able to recycle the original LEDs in the flashlight because the leads were clipped too short, you can use any 2 mA LED, round T1 3mm [e.g. Digikey# 475-1402-ND]
Dynamo keychain flashlight
I used a dynamo LED keychain flashlight which was marked as AIDvantage and made by LTA, Inc. (item# 02119) for this project. There are a variety of these sized flashlights on the market made by different manufacturers -- I've seen them at grocery stores (Giant on the East Coast) and computer stores (Microcenter). You can find them online by Googling: dynamo keychain flashlight. They usually cost less than $5.
I discovered there are some minor variations between flashlights made by different manufacturers. One flashlight I got at Microcenter did not have a circuit board for the LEDs -- the LEDs were just soldered directly to the battery. This LED circuit board is nice but not required. If you find that there's no separate circuit board for the LEDs, you can just solder the respective positive and negative leads of the LED+resistor combo and output cable together. A little hot glue inside the faceplate near the LED and output cable can give the assembly some mechanical strength. The other variation was that leads on the switch on this version were also soldered slightly differently to the battery. Otherwise, it was pretty much identical.
I used a USB A male to mini-B USB male cable scavenged from a dead MP3 player as the output cable. I chose this cable because the mini-USB input is common for small circuits. Since there are 4 connections inside this cable, you have to figure out which wires are the positive & negative leads. However, you can use any output cable type you want if you know the polarity.
To test the circuit, you will also probably want to have the complementary jack available for the output adapter. I de-soldered the mini-B receptacle from the dead MP3 player and connected red and black wires to power 5V and ground pins, respectively.
You will need the following tools to build and test the modified dynamo:
-- wire stripper
-- soldering iron, solder, & flux (this Instructable assumes you have soldered before)
-- voltmeter & test leads
-- small Phillips screwdriver (for opening the flashlight case)
-- electrical tape
-- small wire cutters
-- small pliers
-- tweezers (optional, but recommended)
-- adjustable arms vise, third hand tool (optional, but recommended)
-- small flathead screwdriver (optional, but recommended)
-- hot glue gun (optional, but recommended)
-- hobby knife (optional, but recommended)
Step 3: Circuit Origami: MAX756 and storage capacitor
B. Flip the chip over (i.e. rotate 180 degrees through the long axis) and clip pins 4 and 5. These pins go to the MAX756's low-battery indicator feature and are not used in this Instructable. You could modify the circuit and use these pins to determine when the voltage on the storage capacitor (C1) is low. Flip the storage capacitor over so the negative pin is on the left.
C. Place the MAX756 on the storage capacitor so that the chip is roughly between the storage capacitor's negative C1(-) and positive C1(+) pins.
D. Bend the storage capacitor pins towards the MAX756 as if to clip the chip in place. Bend pins 2 and 7 on the MAX756 so they are nearly touching the storage capacitor's negative pin C1(-). Bend pin 6 so it is nearly touching the storage capacitor's positive pin C1(+).
E. Solder together C1(-) and pins 2 and 7 on the MAX756. Then solder together C1(+) and pin 6 on the MAX756.
F. Finally, cut a small piece of electrical tape roughly the size of the MAX756's height and width. Use this piece to cover the joints soldered in E.
Step 4: Circuit Origami: inductor, reference capacitor, Schottky diode
B. Solder L1 to pins 1 and 8 and clip the remaining L1 lead length.
C. Place ceramic capacitor C4 so that one lead touches pin 3 on the MAX756 and the other is pressing against an exposed part of pin 2, which is now mostly underneath the electrical tape.
D. Solder C4 to pins 2 and 3 and clip the remaining C4 lead length.
E. Looking at MAX756 with pin 1 in the upper left, place the Schottky diode D2 on the ledge created by the large capacitor C1. Bend the D2 cathode D2(-) pin -- identified with a band -- around the MAX756 body so that it's touching the positive terminal of C1, C1(+). Bend the D2 anode D2(+) up so that it's touching pin 8 on the MAX756.
F. Solder the D2 pins to the MAX756 and clip the remaining lead length. Trim pins 8 and 3.
Step 5: Circuit Origami: electrolytic capacitors, part 1
B. Bend C3(-) around C2(-).
C. Solder the two negative leads together close to C2. This will create a ground lead for the two capacitors. Make sure not to accidentally solder the positive terminal of C2. Clip the remaining length of C2(-).
D. Flip the capacitors towards you. Bend C3(-) into the channel create between the two capacitors. Near the end of the capacitors, bend the remaining length 90 degrees like you are creating a foot for the two capacitors.
E. With C1(-) facing you, place C2 and C3 on the left side and tuck the C3(-) foot between the C1(-) terminal and the C1 body.
F. Solder C3(-) to C1(-). You are tying the ground pins of C2, C3, and C1 together.
Step 6: Circuit Origami: electrolytic capacitors, part 2
B. Solder C3(+) to pin 1 on the MAX756. Trim the remaining length of pin 1.
C. Turn the assembly so that it's resting on the negative lead of C1, C1(-). Cut a strip of electrical tape that's narrower than the width of capacitors C2 and C3 together and about twice as long. Place this electrical tape between C1 and C2/C3 so that it will cover the C2/C3 ground pins. This will keep C2(+) from accidentally touching and shorting to ground.
E. Bend C2(+) 90 degrees so that it's over the C2/C3 solder joint. Then bend it 90 degrees towards the C1(+) terminal.
F. Solder C2(+) to C1(+) and trim the remaining length.
Step 7: Making the output cable
Cut the cable about 5 inches from the tip of the mini-B end. Strip the USB-A end and the 4 wires inside. To determine which wires are the positive and ground, plug the USB-A into a powered USB jack. Test combinations of wires with a voltmeter - if there are red and black wires, they probably provide positive power and ground, respectively.
Strip the outer insulator on the mini-B end about 1/4 inch. Once you know which wires are positive and ground, J1(+) and J1(-), strip these wires in the mini-B end and trim the remaining two wires.
Step 8: Disassembling the flashlight
B. The flashlight should pull apart easily. Identify which parts are the case top, case bottom, and faceplate.
C. Pull the electronics out.
D. Clip the two wires close to the faceplate. You will use the wire that is soldered to the switch, so keep that wire as long as possible. Then clip the wire and the end of diode D1 (the negative, cathode end is marked with a black line) close to the stacked coin batteries so that the wire and diode lengths extending from the motor M1 are as long as possible.
Step 9: Preparing the faceplate
A. Wedge a flathead screwdriver between the faceplate plastic and the LED circuit board.
B. Twist the screwdriver. The faceplate and LED circuit board should pop apart.
C. Find the nub on the plastic faceplate.
D. Clip the nub with wire cutters.
E. The nub side will face out in the new dynamo.
F. Desolder the LEDs from the LED circuit board. Try to extract the LEDs intact and leave the holes open for future pins.
Step 10: Making the faceplate
B. Bend the D3 anode D3(+) 90 degrees and insert D3(-) into the hole in the LED circuit board.
C. Trim D3(+) after the bend so that it's less than a 1/8 inch long. Clip one lead of the 1k ohm resistor R1 so that it's also about 1/8 inch long. Feed the long end of R1, R1(2), through the hole in the LED circuit board and solder the short ends of R1 and D3(+) together.
D. Flip the LED circuit board over. Solder R1(2) to the hole unoccupied by D3(+) and trim the remaining length. The strip of copper R1(2) is now soldered to is the positive bus.
E. Flip the LED circuit board back over. Feed the output cable through one of the holes in the plastic faceplate. Note that direction of the faceplate is now inverted and the faceplate will stick out when you're finished.
F. Solder the J1(+) through the hole that connects to the positive bus. Solder J1(-) to the ground bus.
Step 11: Completing the faceplate
B. Since you do not need the coin batteries, desolder a wire from the stack. Solder this wire to R1(2). This wire will provide power to LED and output cable after being connected to the step-up converter output.
Step 12: Installing the switch and step-up converter circuit
B. Make sure the switch pinout looks similar to the photo, with a wire soldered to the top pin SW1(2) and none on the bottom two. Bend the middle pin SW1(1) about 45 degrees away from the switch body. You can clip the bottom pin.
C. The bottom half of the case has three plastic features on the faceplate side which would prevent the new circuit from fitting inside. Trim these using wire cutters.
D. You may need to use a hobby knife to cut these features down flush with the rest of the case.
E. Put the switch into the bottom half of the case in its original location. Make sure the pin with the wire, SW1(2), is closest to the faceplate end.
F. Place the entire step-up converter circuit in the cavity, with the large capacitor C1 facing towards the switch and the two electrolytic capacitors C2 and C3 in the back. SW1(1) should be pressing against the negative terminal of C1, C1(-). If it is not, bend it towards the capacitor. You may want to put some electrical tape on C1(-) behind the SW1(2) pin so that it does not short.
Step 13: Connecting the faceplate and step-up converter circuit
B. Cut and strip the M1(-) wire to the appropriate length and solder the wire, SW1(1), and C1(-) together. This is an important connection, so make sure the three are soldered.
C. Turn the case so that the motor is on your left, and bend the cathode lead of D1, D1(-), so that it's touching an exposed part of the positive terminal of C3, C3(+).
D. Solder D1(-) and C3(+) together and trim the remaining length of D1(-).
E. Solder the SW1(2) wire to the negative bus of the faceplate.
F. Solder the wire connected to the positive bus of the faceplate to the positive terminal of the large capacitor, C1(+).
Step 14: Reassembly
You may want to place some electrical tape on the motor if you think the diode D1 is at risk of shorting to the motor case.
Put the gears and handle back in their original positions. Consult the photo below to see how they are oriented in the case.
Put the top half of the case on top of the bottom half. The two parts should fit closely together if the step-up converter was made pretty closely to the one in this Instructable.
Flip the new and improved power supply over and tighten the four screws.
Step 15: Testing
Hold the dynamo power supply in your left hand and crank the handle with your right hand. Around two rotations per second is good. You should encounter a little resistance -- that's the capacitor charging. After a few seconds the voltage will be high enough the light the LED. As the capacitor approaches 5V, the resistance will drop. At that point, the capacitor is charged.
If you have a complementary adapter with power leads for your output cable, you can connect it to a voltmeter. Around the point where cranking resistance drops you should see that voltage approaches and stays near 5V.
If you encounter resistance but the LED does not light, check the faceplate connections. If the output voltage seriously overshoots 5V, make sure the electrolytic capacitors are correctly soldered. If you do not encounter any resistance and it clearly isn't working, it's possible there's a short somewhere in the step-up converter circuit.
Step 16: Application
I also tested the dynamo supply with a hobby motor. While cranking, the motor was humming along with 50 mA of current.