Super LED Blinking With 1 Transistor | Make 12-24 Volt Flashing Lights
In this Instructable, I am going to try and explain, step by step, how I designed this Ultra Low-Power LED Flasher with the following specification:.
LED flash rate 1 flash per second
LED on-time of 5ms
At least 2 years battery life
Use standard "AA" size batteries
Unfortunately, uses two "AA" batteries
Easy obtainable components
No surface mount components
No PC Board
Before I decided on a baseline, I tested several common flasher circuits. These included standard flashing LED's, the NE555 timer IC, and transistor flip-flop circuits commonly used. The first problem I experienced, is the amount of components needed to get a short output pulse of around 1 - 5ms every 1 second, or On/Off ratio of 1/1000 to 5/1000. Thus I did not investigate these designs further.
The circuit I decided to use as a starting point, is the LM3909 LED flasher IC. Although production of this very popular chip has ended, they are still freely available on eBay at a reasonable cost. Looking at the LM3909 datasheet, it states that the flasher circuit can run up to 6 months on a single alkaline cell.
So why did I not use the LM3909, you may ask? Well, in South Africa, our postal system had collapsed, and the average waiting time for an eBay order is between 3 and 6 months....
My Baseline:
Standard AA battery
12 Months minimum operation per battery
Replace the LM3909 with a micro controller
I decided to make use of an AtTiny85 micro controller, as I have some in my bins. Using the datasheet, I had two options:
AtTiny85-20
Minimum operating voltage is 2.7V
AtTiny85V-10
Minimum operating voltage is 1.8V
From above data, it is clear that I will have to use two AA size batteries to power the circuit. So the operating voltage will be taken as nominal 3V for the rest of the design.
The next step was to decide on which AtTiny to use, and for this, I had to do some research on battery capacity.
I will be basing all my calculations on the “AA” Duracell MN1500, as this is what I had on hand. This might not be the best battery for the project.
Minimum operating voltage of the AtTiny are taken, and divided by 2 to give minimum voltage per cell required.
AtTiny85-20
Vminimum = 2.7V, or 1.35V per cell
From the datasheet (lowest mW curve available), battery capacity is around 0.25Ah at 1.35V.
AtTiny85V-10
Vminimum = 1.8V, or 0.9V per cell
From the datasheet (lowest mW curve available), battery capacity is around 2.2Ah at 0.9V.
Conclusion
From above findings, it shows that by using the AtTiny85V-10, the operating time on a set of batteries can be greatly increased.
Next, we need to determine the average current consumption of our circuit. Using the Duracell MN1500 with an AtTiny85V-10, we have 2.2Ah available.
Ah = Iaverage x 24 hours x 365 days,
2.2Ah = Iaverage x 24 x 365
Iaverage = 251uA
Conclusion
The flashing circuit should use less than 251uA average to get a battery life of one year.
The test circuit is per schematic, and to keep measurements comparable, I connected the circuit to a 3V regulated power supply.
The test code is straight forward, switching an output on for 5ms, and off for 995ms.
In the Arduino IDE, there are only three internal clock selections available. I will test each of them individually. To learn more about setting the configuration bits, please refer to AVRDude.
The AtTiny can be placed into a sleep mode when the LED is in the off state. This will shut down certain hardware in the chip, as well as further reducing the average current used.
For the design, I want the LED to flash once every second. By using the SLEEP commands, I will turn on the LED for 5ms, then turn off the LED. Thereafter, the chip will be placed into the SLEEP mode to reduce power.
After 1 second, the build-in watchdog timer will be used to wake up the chip from sleep, and the LED will be turned on again.
For this, I will be using additional libraries
Three new functions were added to the program
setup_watchdog()
With this function, the time-out of the watchdog timer is set to 1 second
system_sleep()
In this routine, the chip is placed into SLEEP mode, and will wait for a time-out of the watchdog timer to wake up.
ISR(WDT_vect)
This is the interrupt vector where the code will jump to once the watchdog timer times out. After the watchdog timer timed out, the code will resume to run again.
Findings
With SLEEP mode enabled, the current readings are as follow:
Imax = 0.32mA
Imin = 0.24mA
Iavg = 0.27mA, or 270uA
Conclusion
Even by placing the chip into SLEEP mode, it does not reduce the average current to a low enough value.
There are still a few more things that can be done to reduce the power in SLEEP mode
I/O Pin Configuration
When a pin is set up as an OUTPUT, it uses internal circuitry to switch the pin HIGH or LOW. This uses power. The solution might be to switch the pin to an INPUT during SLEEP to save extra power.
By configuring OUTPUT pins as INPUTS dDuring SLEEP, and turning of the A/D converter, the current readings are as follow:
Imax = 61.2uA
Imin = 4.4uA
Iavg = 8.7uA
Conclusion
The aim was to get an average current of less than 251uA. We are already on 8.7uA.
With an average current of say 10uA for the AtTiny85, almost all power can now be directed towards driving the LED.
Lastly, by changing some High Byte fuses, current consumption can be decreased slightly further.
Set Fuse Low Byte = 0x42
Set Fuse High Byte = 0xFF
The 100K pull-up resistor on the RESET pin can now be removed from the circuit.
! ! ! NOTE! ! !
By setting the High Byte to 0xFF, will disable the external RESET pin, as well as the Serial Programming circuit.. Only use this additional configuration if you are able to reset the AtTiny config bits with a separate programmer.
By disabling the Serial Download Enabled bit, and by disabling the external RESET function, the final readings are:
Imax = 56.0uA
Imin = 4.0uA
Iavg = 8.1uA
Conclusion
With an average current of say 10uA for the AtTiny85, almost all power can now be directed towards driving the LED.
The goal is to use an average of 251uA. With the Attiny85 using only 10uA, we are left with
251uA - 10uA, or 240uA
Assuming a LED current of 20mA, we can now calculate the On time
Ton = 240uA per second / 20mA per second
Ton = 12 ms
The code was changed to flash the LED for 12ms.
LED Resistor
First, we need to determine the forward voltage drop of the LED at 20mA. I will be using a super-bright red LED, with part number WW05A3SRP4-N.
At 20mA, the forward voltage drop of the LED was measured to be 1.993V.
Thus, Rled should be = (3V - 1.993V) / 20mA, or 50R. Use 56R.
Or will it be 56R???
Findings
Iavg = 140uA, well below the calculated 250uA.
Using a scope (blue trace), the voltage across the resistor was measured to calculate the LED current.
Vres = 640mV, and measured value of R = 55.9R
Thus Iled = 640mV / 55.9R, or 11.4mA.
Conclusion
Sometimes, calculated versus actual values does not correspond. Do not accept this, Find out where the discrepancy comes from.
Looking at Table 21-1 of the AtTiny datasheet, I found my answer :) :). I assumed the output voltage on AtTiny pin will be 3V when turned on. However, the datasheet states that the output voltage can be as low as 2.5V when operating at 3V (last line in the table).
On the scope yellow trace, the pin output voltage can be seen as 2.6V when the LED is on.
Calculate the final value of R
Vpin = 2.60V, Vled = 1.993V @ 20mA
R = (2.6V - 1.993V) / 20mA, or 30.4R, use 33R
Iled then calculated as (2.6V - 1.993V) / 33R, or 18mA.
Findings
LED is on for only 12ms every second, so Iavg = 18mA x (12ms/1000ms), or 216uA
Add the 10uA, and total current consumption is around 230uA
Now, 230uA
So, the battery lifetime will be 2.2Ah / 2.02Ah, = 1.09 years.
Add some buffer, and set the On Time to 10ms.
Super LED Blinking With 1 Transistor | Make 12-24 Volt Flashing Lights
In this Instructable, I am going to try and explain, step by step, how I designed this Ultra Low-Power LED Flasher with the following specification:.
LED flash rate 1 flash per second
LED on-time of 5ms
At least 2 years battery life
Use standard "AA" size batteries
Unfortunately, uses two "AA" batteries
Easy obtainable components
No surface mount components
No PC Board
Before I decided on a baseline, I tested several common flasher circuits. These included standard flashing LED's, the NE555 timer IC, and transistor flip-flop circuits commonly used. The first problem I experienced, is the amount of components needed to get a short output pulse of around 1 - 5ms every 1 second, or On/Off ratio of 1/1000 to 5/1000. Thus I did not investigate these designs further.
The circuit I decided to use as a starting point, is the LM3909 LED flasher IC. Although production of this very popular chip has ended, they are still freely available on eBay at a reasonable cost. Looking at the LM3909 datasheet, it states that the flasher circuit can run up to 6 months on a single alkaline cell.
So why did I not use the LM3909, you may ask? Well, in South Africa, our postal system had collapsed, and the average waiting time for an eBay order is between 3 and 6 months....
My Baseline:
Standard AA battery
12 Months minimum operation per battery
Replace the LM3909 with a micro controller
I decided to make use of an AtTiny85 micro controller, as I have some in my bins. Using the datasheet, I had two options:
AtTiny85-20
Minimum operating voltage is 2.7V
AtTiny85V-10
Minimum operating voltage is 1.8V
From above data, it is clear that I will have to use two AA size batteries to power the circuit. So the operating voltage will be taken as nominal 3V for the rest of the design.
The next step was to decide on which AtTiny to use, and for this, I had to do some research on battery capacity.
I will be basing all my calculations on the “AA” Duracell MN1500, as this is what I had on hand. This might not be the best battery for the project.
Minimum operating voltage of the AtTiny are taken, and divided by 2 to give minimum voltage per cell required.
AtTiny85-20
Vminimum = 2.7V, or 1.35V per cell
From the datasheet (lowest mW curve available), battery capacity is around 0.25Ah at 1.35V.
AtTiny85V-10
Vminimum = 1.8V, or 0.9V per cell
From the datasheet (lowest mW curve available), battery capacity is around 2.2Ah at 0.9V.
Conclusion
From above findings, it shows that by using the AtTiny85V-10, the operating time on a set of batteries can be greatly increased.
Next, we need to determine the average current consumption of our circuit. Using the Duracell MN1500 with an AtTiny85V-10, we have 2.2Ah available.
Ah = Iaverage x 24 hours x 365 days,
2.2Ah = Iaverage x 24 x 365
Iaverage = 251uA
Conclusion
The flashing circuit should use less than 251uA average to get a battery life of one year.
The test circuit is per schematic, and to keep measurements comparable, I connected the circuit to a 3V regulated power supply.
The test code is straight forward, switching an output on for 5ms, and off for 995ms.
In the Arduino IDE, there are only three internal clock selections available. I will test each of them individually. To learn more about setting the configuration bits, please refer to AVRDude.
The AtTiny can be placed into a sleep mode when the LED is in the off state. This will shut down certain hardware in the chip, as well as further reducing the average current used.
For the design, I want the LED to flash once every second. By using the SLEEP commands, I will turn on the LED for 5ms, then turn off the LED. Thereafter, the chip will be placed into the SLEEP mode to reduce power.
After 1 second, the build-in watchdog timer will be used to wake up the chip from sleep, and the LED will be turned on again.
For this, I will be using additional libraries
Three new functions were added to the program
setup_watchdog()
With this function, the time-out of the watchdog timer is set to 1 second
system_sleep()
In this routine, the chip is placed into SLEEP mode, and will wait for a time-out of the watchdog timer to wake up.
ISR(WDT_vect)
This is the interrupt vector where the code will jump to once the watchdog timer times out. After the watchdog timer timed out, the code will resume to run again.
Findings
With SLEEP mode enabled, the current readings are as follow:
Imax = 0.32mA
Imin = 0.24mA
Iavg = 0.27mA, or 270uA
Conclusion
Even by placing the chip into SLEEP mode, it does not reduce the average current to a low enough value.
There are still a few more things that can be done to reduce the power in SLEEP mode
I/O Pin Configuration
When a pin is set up as an OUTPUT, it uses internal circuitry to switch the pin HIGH or LOW. This uses power. The solution might be to switch the pin to an INPUT during SLEEP to save extra power.
By configuring OUTPUT pins as INPUTS dDuring SLEEP, and turning of the A/D converter, the current readings are as follow:
Imax = 61.2uA
Imin = 4.4uA
Iavg = 8.7uA
Conclusion
The aim was to get an average current of less than 251uA. We are already on 8.7uA.
With an average current of say 10uA for the AtTiny85, almost all power can now be directed towards driving the LED.
Lastly, by changing some High Byte fuses, current consumption can be decreased slightly further.
Set Fuse Low Byte = 0x42
Set Fuse High Byte = 0xFF
The 100K pull-up resistor on the RESET pin can now be removed from the circuit.
! ! ! NOTE! ! !
By setting the High Byte to 0xFF, will disable the external RESET pin, as well as the Serial Programming circuit.. Only use this additional configuration if you are able to reset the AtTiny config bits with a separate programmer.
By disabling the Serial Download Enabled bit, and by disabling the external RESET function, the final readings are:
Imax = 56.0uA
Imin = 4.0uA
Iavg = 8.1uA
Conclusion
With an average current of say 10uA for the AtTiny85, almost all power can now be directed towards driving the LED.
The goal is to use an average of 251uA. With the Attiny85 using only 10uA, we are left with
251uA - 10uA, or 240uA
Assuming a LED current of 20mA, we can now calculate the On time
Ton = 240uA per second / 20mA per second
Ton = 12 ms
The code was changed to flash the LED for 12ms.
LED Resistor
First, we need to determine the forward voltage drop of the LED at 20mA. I will be using a super-bright red LED, with part number WW05A3SRP4-N.
At 20mA, the forward voltage drop of the LED was measured to be 1.993V.
Thus, Rled should be = (3V - 1.993V) / 20mA, or 50R. Use 56R.
Or will it be 56R???
Findings
Iavg = 140uA, well below the calculated 250uA.
Using a scope (blue trace), the voltage across the resistor was measured to calculate the LED current.
Vres = 640mV, and measured value of R = 55.9R
Thus Iled = 640mV / 55.9R, or 11.4mA.
Conclusion
Sometimes, calculated versus actual values does not correspond. Do not accept this, Find out where the discrepancy comes from.
Looking at Table 21-1 of the AtTiny datasheet, I found my answer :) :). I assumed the output voltage on AtTiny pin will be 3V when turned on. However, the datasheet states that the output voltage can be as low as 2.5V when operating at 3V (last line in the table).
On the scope yellow trace, the pin output voltage can be seen as 2.6V when the LED is on.
Calculate the final value of R
Vpin = 2.60V, Vled = 1.993V @ 20mA
R = (2.6V - 1.993V) / 20mA, or 30.4R, use 33R
Iled then calculated as (2.6V - 1.993V) / 33R, or 18mA.
Findings
LED is on for only 12ms every second, so Iavg = 18mA x (12ms/1000ms), or 216uA
Add the 10uA, and total current consumption is around 230uA
Now, 230uA
So, the battery lifetime will be 2.2Ah / 2.02Ah, = 1.09 years.
Add some buffer, and set the On Time to 10ms.
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