#1

One of the most common electronics starter projects is the Simple Blinking / Flashing LED Circuit. You can find references to this project and diagrams on many websites. I have created a simple BOM (Bill of Materials) that you can get here at Digi-Key to build this circuit. Below is a simple schematic I have built using Digi-Key’s Scheme-it tool. (Values calculated using a 9VDC power supply or battery.)

Desc P/N QTY
PNP Transistors PN2907A-APCT-ND Qty 2
Resistor 300 Ohms 300QBK-ND Qty 2
Resistor 100k Ohms 100KQBK-ND Qty 2
Cap 10 uF 732-8820-1-ND Qty 2
LED C503B-GCN-CY0C0791-ND Qty 2

Here is a Cart that has the above components in it for your convience.

Some notes for this circuit.

• Changing the Capacitor Values will change the speed at which the LEDs blink.
• If you change the LEDs in this circuit you will need to calculate the value of R2 & R3.

If you need here are some of the prototype products used in this build.
Jumper Wires BKWK-3-ND
9V Battery P687-ND

#2

I have a couple of questions about this circuit. I would like to have a power source that is 18-19.8 volts, instead of the 9 volts in this circuit.

I understand that the LEDs have to be protected with higher resistance resistors for R2 and R3 and I know how to calculate that resistance and the wattage as well. My question is what about R1 and R4. Do the resistance values of these resistors need to change if the voltage is increased to 18-19.8 volts? If so, how do you calculate the new resistance values.

It would appear that from the parts list that most of the parts can handle the higher voltage. Is there any other parts that would have to be changed to handle the higher voltage?

You mention that the flashing frequency will change if you change the values of the capacitors. Is there a way to calculate the flashing frequency of this circuit?

Thank you

#3

Hello @JoeZastrow

You would not need to change the resistance, however you would need to do some current calculations on the circuit for your new input voltage to ensure the resistors are all rated to handle this higher voltage drops. You may also need to look at the transistors as the ones I am using have a very low max power.

As far as calculating the flashing frequency you would need to calculate the discharge time of the capacitor in relation to the turn on / turn off voltages on the transistors that you are using.

-Robert

#4

LED blinker is the most common starter project in MCU programming as well. It is implemented on numerous MCUs using all possible schematics and algorithms. Here at Qoitech we have it as a reference benchmark for AVR8, STM32 to name just a few. I believe it’s almost the same with pure HW implementations: different types of multivibrators, oscillators, what not.
What am I curious about is energy consumption of this and similar HW-only implementations. It could be less than for MCU-based because there’s no timer running between LED toggles. On the other hand there still are leakages and other types of loss.
@Robert_Fay: have you ever had a chance to record current consumption for this or similar implementation?

#5

@Yabusame I took the parts from the post above into the lab this morning and on the breadboard I was running between 4.9mA and 5.1mA at 9V.

#6

Thanks! That sounds amazing. I checked the archives and see that for Xplained Pro board with ATmega324PB on it, running at 1.9 V current consumption is between 500 and 900 uA (see the screenshot).
Of course, it depends on the LED. Voltage could be lower for HW-only implementation, too.