User talk:Daveb
Dave Brown - hacker in spirit.
Project Ideas
| Project | Status | Notes |
| Reconfigurable sailing proa | plan | Rob Denny at http://www.harryproa.com has some interesting things to say about Proas and how they could be the basis for a cheap, high performance, spacious sailing cruising boat. Perhaps the ideal cruising boat :). I want to explore the sailing characteristics in a model. |
| LED lights and lenses for my bike | plan | http://pilom.com/BicycleElectronics/DynamoCircuits.htm explains the matching of LEDs to bike generators. I plan to have several strings of LEDs with different beam angles and aiming points - and the appropriate string of LEDs turned on for the curent bike speed.
Success criteria:
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Project Status
Bike Headlight
Have ordered LEDs, reflectors and lenses. Have an attiny85 from the space components. Still need a dynamo, some diodes, mosfets, npns, a circuit and some firmware - oh and maybe a heatsink for the LEDs:). The project is moving along.
Have received the LEDs (white Lambertian (but not the reflectors etc). Have a bottle dynamo, but dont know much about it yet.
Still thinking about what the maximum voltage for the LED strings will be ..
At 0.7A and 25deg junction temp, voltage per LED varies between 3.03(min), 3.7(typ), 4.47(max). And 3.9V(typ) at 1A current.
Some uncertaintity - current might just get to 0.5 A but I am sure the junction will be hotter than 25C, and see but no indication of change in V drop with temp in the datasheet :(.
Assume 4.0V per LED - gives 16V for 4 LEDs, and 24V for 6 LEDs.
Am expecting 4x30deg, 3x15deg and 3x5deg reflecters.
Originally thinking of four LED strings 1 - very low speed - just a 120 deg med power LED (with voltage doubler to get the reqd V) 2 - low speed - one 30deg 3 - med speed - one 30deg, one 15deg 4 - high speed - one 15deg, two 5deg
Of course, the length of the strings depend on the V generated by my bike speed.. not really predictable/
Simplify (only last 3 strings) and no V doubler.
Switches for LED current
- This SMD Dual Logic Level MOSFET will control two sets of LEDs directly from the MCU - given enough heat-sink capability on the board and enough voltage from the MCU (3.3 or 5v compatibke , Has ESD, over temp and over current protection... is really tiny! If the logic level REALLY needs to be 3.3v then that is an issue :( 3.3v logic turns into about +3.45v regulator (3.3v + a 200mV diode drop to allow the MCU tp be reprogramed in circuit) with 415mV regulation margin - nearly 4v reqd for the MCU.. when the LEDs start to light up near 3v :(.
- This TO220 MOSFET should carry the 0.5a curent - perhaps without any heatsinking at all. and combined with a small transister should operate well at a supply voltage of 3v.
- This PNM34UN is 2A mosfet, with Rds of 0.08 at 1.8V throu to about 0.05 at 4.5V; max Vds of 30V; £0.16 each. there are several similar parts at Vds 30V for a bit more money.
- PMN27UN is similiar to the above; but 20V max Vds with a bit smaller Rds(on) - also marginally cheaper.
There are a few impacts of this choice..
- SMD or through hole!!!
- Many fewer devices from the SMD integrated logic level MOSFETS
- Higher operating voltages might be required by the SMD MOSFETs
I need to establish the maximmum operating voltage!
Voltage regulator for the MCU
Needs to operate of a widely varied supply voltage. The dynamo generates a voltage proportional to the bike speed. Potentially reaching hundreds of volts. And the higher the voltage climbs, the more power I can use in the LEDs! Many components to deal with 0.5A are specced at about 40 to 60V - so that is a very high limit for the working voltage. A chain of 6 LEDs would need 24v. My plans are for 3 LEDs in a chain - so 20V is a good target. MCU Vcc of 3V (supporting 10MHz clock) these are some possible regulators.
- 3.0v, 200mV drop out, 50 mA, max 40v supply from a 3.2V diode bridge supply :). It is a 4 pin SMD device for £1.60.
- 3.0v, 120mV drop out, 50mA, 16V max supply will do above from a 3.12V supply, but restricts me to chains of fewer than four LEDs. £0.46
- 3v, 350mV drop out, 100mA, 30V max supply is a small 3 pin through hole package for £0.46. Required voltage is now 3.35V :(.
MCU Vcc of 2.5V (max clock of 8Mhz)
- a 2.5V, 300mV drop out, 100mA, 20V max power supply will allow operation at 2.8V:).
Diode to isolate MCU
This diode should allow the MCU to be reprogrammed in circuit, without blowing up the vreg or anything else with badly applied voltages.
It is a dual common cathode part.
- 1a, 160mV fwd voltage drop, 30v max reverse voltage £0.154 - however this wonder component is US stock, so £16 extra charge! (there are a few similar low voltage drop diodes and they are all US stock).
- The Datasheet says the VF Max is 0.4V - crap !
20V, 30V or 60V diode bridge.
From what I read yesterday, the voltage drop of a schottky diode is a function of the junction temp, the current passed, the reverse voltage barrier, and the size of the diode junction (which I guess becomes cost). There is this £3.19 diode 9A/15V that has a Vf for 1A at 25C less than 250mV. If you could heat the junction to 100C it would be about .1V!! AT 100mA it is less than 200mV Vf.
Since the voltage out of the dynamo is essentially a function of the road speed, it could be quite high. leading to my difficulties in finding low Vf diodes..
If I put a pair of zener diodes across the output of the dynamo - before the rectifier, then the rest of the circuit is limited to only that voltage, and the diodes in the bridge recifier are limited to half of that.
I am not sure if 20V or 30V is the right target voltage for the rectifier. Higher voltages get more complex, as larger logic level MOSFETs seem to need 5.5 to 4.5V gate voltages rather than 3V, 2.5V or even 1.8V. Also, low-drop-out v-reg that will take > 30V as input are rare.
I am leaning towards 20V (or 18V to split the diff between LEDs and specs).
If 20V then 10V/1A seems a useful diode to rectify 0.5A with low losses.
MCU programming
All AVR MCUs come with debugWire disabled :( - see Connecting to a board with JTAG MK2.
To enable debugWire I need to use ISP/ which needs MISO,MOSI,SCK and RESET, VCC, and GND! Can do with the DIP by programming before putting in the board; using a socket.
I wonder how the users of SMD packages manage it?
rough design
- dynamo->AC
- AC->2 crossed 5% 19V zener diodes to clip(+/- 5% -> 18 to 20V output) to safe range of rest of system
- AC->diode bridge->DC (using 10-15V diodes, loose .5V so 17.5 to 19.5 max V)
- (three of)DC->LED strings->logic level 20V mosfets->GND
- DC->2.5V reg->Vcc->MCU->GND (requires DC >= 2.8V)
- AC->diode-> 100k resister-(MCU int0 pin)->2.5V zener diode->GND
- MCU_reset->Reset_debugWire->100k resistor->Vcc
- MCU_[3*gpios]->MOSFET Gates
- MCU_[gpio]->light sensor
- DC->prog supply pin
- Vcc->Vref_debugWire
- Gnd->Gnd_debugWire
NOTES
If the LEDs light up below 2.8V, then I need to think about lower voltage supply for MCU.
One LED string should default to ON - so that while the MCU is booting there is a power consumer to stop capacitors charging to unsafe voltages.
Not sure what the failure modes are and what protection should be added.
Plans for cooling of LEDs are still vaugue.
