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Step 2Power Management
Power Management
For a project like this it's obvious that portability should be a key aspect. Batteries are thus the logical choice!
Now, for portable devices it is fairly key that you pick a battery source that is either rechargeable or easily available. The two main options are the 9V PP3 battery or AA batteries. I'm sure some people will assume that a 9V battery is the best option because hey, 9V is better than 3 right?
Well, not in this case. 9V batteries whilst very useful, produce their voltage at the expense of battery life. Measured in mAh (milliamp hours), this rating tells you in theory how long a battery will last operating at 1mA in hours (though take it with a pinch of salt, these are often under ideal, low load conditions). The higher the rating, the longer the battery will last. 9V batteries are rated at up to and around 1000mAh. Alkaline AA's on the other hand have almost three times as much at 2900mAh. NiMH rechargables can reach this, though 2500mAh is a reasonable amount (note that rechargeable batteries operate at 1.2V not 1.5!).
The LCD screen needs a 5V input (ý10%) and the AVR (the microcontroller) needs roughly the same (though it can go as low as 2.7 for low frequency clock speeds). We also need a fairly stable voltage, if it fluctuates about it could cause problems with the microcontroller.
To do this we'll use a voltage regulator, you need to make a choice over price vs efficiency now. You've got the option of using a simple 3-pin voltage regulator like the LM7805 (78 series, +5 volts output) or a small integrated circuit.
Using a simple regulator
If you choose to go with this option, you need to bear a few points in mind. Firstly, three pin regulators almost always need an input that is higher than their output. They then step the voltage down to the desired value. The downside is that they have awful efficiency (50-60% is good going). The upside is that they're cheap and will run with a 9V battery, you can pick up a basic model for 20 pence in the UK. You should also bear in mind that regulators have a dropout voltage - the minimum gap between input and output. You can buy special LDO (Low DropOut) regulators that have dropouts at around 50mV (compared to 1-2V with other designs). In other words, look out for LDOs with a +5V output.
Using an integrated circuit
The ideal way to go is a switching regulator. These will be, for our purpose, normally 8-pin packages that take in a voltage and give us a regulated output at a high efficiency - almost 90% in some cases. You can get step up or step down converters (boost/buck respectively) depending on what you want to put in, alternatively you can buy regulators that will take either above or below the desired output.
The chip i'm using for this project is a MAX619+. It is a 5V step up regulator that takes 2 AA's (the input range is 2V-3.3V) and gives a steady 5V out. It only needs four capacitors to operate and is very space efficient. Cost - ý3.00 including the caps. Arguably it's worth the splurge just to get a bit more use out of your batteries. The only major downside is that it's not short circuit protected, so if there is a current surge, be warned! This is reasonably trivial to fix with an add on circuit however:
Another useful chip design - although not nearly as neat a solution is the LT1307. Again, a 5V regulator, but it can take a variety of inputs and has useful things like low battery detection. It costs quite a bit more at nearly ý5 with inductors, large capacitors and resistors.
Voltage Rails
We are going to be using two main voltage rails (plus a common ground). The first will be the 3V from the battery, this will be used to power the LEDs and other relatively high power components. My MAX619 is only rated up to 60mA (though absolute maximum is 120mA) so it is easier to connect the microcontroller to a MOSFET to control any LEDs. The MOSFET draws almost no current and acts as a break in the circuit when the gate input is under around 3V. When the microcontroller sends out logical 1 on the pin, the voltage is 5V and the FET turns on, then just acting as a short circuit (i.e. a piece of wire).
The 5V rail will power the LCD, Microcontroller and any amplification circuits for input sensors.
Power Consumption
If we look at various datasheets, we note that the AVR takes no more than 15-20mA at maximum load.
The LCD takes only 1mA to operate (at least when i tested, budget for 2). With the backlight on, it's really up to you to decide. Connecting it straight up to the 5V rail (i tried) is fine, but make sure it has an onboard resistor (follow the traces on the PCB) before you do. It drew 30mA that way - terrible! With a 3.3k resistor it's still viewable (perfect for astro photography) and only draws 1mA. You can still get decent brightness using a 1k or otherwise. I'm fine with mine drawing just under 2mA with the backlight on! If you want, it's trivial to add a brightness knob using a 10k potentiometer.
The IR LED might take 100mA maximum, but i have had good results with 60mA across mine (experiment!). You can then halve that current because you're effectively running at a 50% duty cycle (when the LED is modulated). Anyway, it's only on for a fraction of a second so we don't need to worry about this.
The other LEDs you should play about with, you may find that only a 10mA current is enough to give you a good brightness - certainly look for low power LEDs (excluding the IR one), youre not designing a torch! I chose not to add a power indicator in my circuit, simply because it's a lot of current draw for not much use. Use the on/off switch to check if it's on!
In total, you shouldn't be running more than 30mA at any one time and with a theoretical supply of around 2500 (allowing for variation) mAh that should give you well over 80 hours straight with everything on. With the processor idled for most of the time this will at least double/triple, so you shouldn't have to change your batteries very often.
Conclusion
There we go, that was easy wasn't it! You can either go cheap and cheery with a 9V battery and a LDO regulator at the expense of efficiency or pay a bit more and use a dedicated IC to do it. My budget was still under ý20 even WITH the IC, so you can drop it even further if you need to.
For a project like this it's obvious that portability should be a key aspect. Batteries are thus the logical choice!
Now, for portable devices it is fairly key that you pick a battery source that is either rechargeable or easily available. The two main options are the 9V PP3 battery or AA batteries. I'm sure some people will assume that a 9V battery is the best option because hey, 9V is better than 3 right?
Well, not in this case. 9V batteries whilst very useful, produce their voltage at the expense of battery life. Measured in mAh (milliamp hours), this rating tells you in theory how long a battery will last operating at 1mA in hours (though take it with a pinch of salt, these are often under ideal, low load conditions). The higher the rating, the longer the battery will last. 9V batteries are rated at up to and around 1000mAh. Alkaline AA's on the other hand have almost three times as much at 2900mAh. NiMH rechargables can reach this, though 2500mAh is a reasonable amount (note that rechargeable batteries operate at 1.2V not 1.5!).
The LCD screen needs a 5V input (ý10%) and the AVR (the microcontroller) needs roughly the same (though it can go as low as 2.7 for low frequency clock speeds). We also need a fairly stable voltage, if it fluctuates about it could cause problems with the microcontroller.
To do this we'll use a voltage regulator, you need to make a choice over price vs efficiency now. You've got the option of using a simple 3-pin voltage regulator like the LM7805 (78 series, +5 volts output) or a small integrated circuit.
Using a simple regulator
If you choose to go with this option, you need to bear a few points in mind. Firstly, three pin regulators almost always need an input that is higher than their output. They then step the voltage down to the desired value. The downside is that they have awful efficiency (50-60% is good going). The upside is that they're cheap and will run with a 9V battery, you can pick up a basic model for 20 pence in the UK. You should also bear in mind that regulators have a dropout voltage - the minimum gap between input and output. You can buy special LDO (Low DropOut) regulators that have dropouts at around 50mV (compared to 1-2V with other designs). In other words, look out for LDOs with a +5V output.
Using an integrated circuit
The ideal way to go is a switching regulator. These will be, for our purpose, normally 8-pin packages that take in a voltage and give us a regulated output at a high efficiency - almost 90% in some cases. You can get step up or step down converters (boost/buck respectively) depending on what you want to put in, alternatively you can buy regulators that will take either above or below the desired output.
The chip i'm using for this project is a MAX619+. It is a 5V step up regulator that takes 2 AA's (the input range is 2V-3.3V) and gives a steady 5V out. It only needs four capacitors to operate and is very space efficient. Cost - ý3.00 including the caps. Arguably it's worth the splurge just to get a bit more use out of your batteries. The only major downside is that it's not short circuit protected, so if there is a current surge, be warned! This is reasonably trivial to fix with an add on circuit however:
Another useful chip design - although not nearly as neat a solution is the LT1307. Again, a 5V regulator, but it can take a variety of inputs and has useful things like low battery detection. It costs quite a bit more at nearly ý5 with inductors, large capacitors and resistors.
Voltage Rails
We are going to be using two main voltage rails (plus a common ground). The first will be the 3V from the battery, this will be used to power the LEDs and other relatively high power components. My MAX619 is only rated up to 60mA (though absolute maximum is 120mA) so it is easier to connect the microcontroller to a MOSFET to control any LEDs. The MOSFET draws almost no current and acts as a break in the circuit when the gate input is under around 3V. When the microcontroller sends out logical 1 on the pin, the voltage is 5V and the FET turns on, then just acting as a short circuit (i.e. a piece of wire).
The 5V rail will power the LCD, Microcontroller and any amplification circuits for input sensors.
Power Consumption
If we look at various datasheets, we note that the AVR takes no more than 15-20mA at maximum load.
The LCD takes only 1mA to operate (at least when i tested, budget for 2). With the backlight on, it's really up to you to decide. Connecting it straight up to the 5V rail (i tried) is fine, but make sure it has an onboard resistor (follow the traces on the PCB) before you do. It drew 30mA that way - terrible! With a 3.3k resistor it's still viewable (perfect for astro photography) and only draws 1mA. You can still get decent brightness using a 1k or otherwise. I'm fine with mine drawing just under 2mA with the backlight on! If you want, it's trivial to add a brightness knob using a 10k potentiometer.
The IR LED might take 100mA maximum, but i have had good results with 60mA across mine (experiment!). You can then halve that current because you're effectively running at a 50% duty cycle (when the LED is modulated). Anyway, it's only on for a fraction of a second so we don't need to worry about this.
The other LEDs you should play about with, you may find that only a 10mA current is enough to give you a good brightness - certainly look for low power LEDs (excluding the IR one), youre not designing a torch! I chose not to add a power indicator in my circuit, simply because it's a lot of current draw for not much use. Use the on/off switch to check if it's on!
In total, you shouldn't be running more than 30mA at any one time and with a theoretical supply of around 2500 (allowing for variation) mAh that should give you well over 80 hours straight with everything on. With the processor idled for most of the time this will at least double/triple, so you shouldn't have to change your batteries very often.
Conclusion
There we go, that was easy wasn't it! You can either go cheap and cheery with a 9V battery and a LDO regulator at the expense of efficiency or pay a bit more and use a dedicated IC to do it. My budget was still under ý20 even WITH the IC, so you can drop it even further if you need to.
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This does not help to build it.
I'm not sure what your problem is? This step deals purely with a discussion of the power supply. The later steps show you what should be connected to what.
Buggerit, why has the text gone!