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Topic: Programmable Police LED Flasher (Read 11577 times) previous topic - next topic

hesam_m

The STM8S001J3 is an 8-bit microcontroller that offers 8 Kbytes of Flash program memory, plus an integrated true data EEPROM. It is referred to as a low-density device in the STM8S microcontroller family. This MCU offered in a small SO8N package. In this article, we are going to build a programmable Police LED Flasher device that can be used for vehicles, motorcycles, and bicycles.


YouTube: https://www.youtube.com/watch?v=v0_3puzW-wk


Circuit Analysis

Figure 1 shows the schematic diagram of the device. The heart of this circuit is an STM8S001 microcontroller.



Figure 1

Schematic diagram of the programmable police LED-Flasher


Let's start the analysis from the power supply unit. C2 and C3 are used to reduce the input voltage noises. Then the voltage is handled to the 78M09 [1] (REG1) regulator. It is used to stabilizes the voltage at 9V. C4 and C6 are used to reduce the regulator's output noises.

The output of the REG1 is handled to a first-order RC filter (R28 and C5). It helps to reduce the noises even further because this device might be used continuously in a noisy environment such as a vehicle. The best way to examine the behavior of this filter (or other filter types) is to perform a practical measurement. The SDS1104X-E oscilloscope introduced a nice bode plot feature that can perform this useful calculation.

REG2 [2] is used to convert 9V to 5V to supply the STM8s001 MCU [3] (IC1). C7 is a supplementary filtering capacitor for the IC1.

IC1 MCU is programmed using a single SWIM wire. It stands for the Single-Wire Interface Module. It is a high-speed link between the MCU and the programmer/debugger. This pin must be connected to the SWIM pin of the programmer/debugger. The ground pin must also be connected. This completes the connection (P2).

IC2 and IC3 are logic N-Channel SMD Mosfets [4] that are used to turn on/off the LEDs. The gate pins of both MOSFETs have been pulled down using 4.7K resistors to avoid unwanted triggering (R13, R14). SW1 is a tactile push button that is used to switch between flasher programs. R27 is a pull-up resistor and C8 reduces the possible push-button debouncing noises.

R1 to R26 resistors are used to limit the LEDs current. In each part, I have put 3 LEDs in series that are connected to the +9V rail (Figure 2). The characteristics of LEDs vary from manufacturer to manufacturer. Therefore we can not assign a fixed limiting series resistor for all circumstances. The maximum tolerable current of a 5mm LED is around 25mA. Therefore the resistor value that could limit the current to somewhere around 15mA (a bit higher than a half) looks sufficient and does not affect the LEDs' lifetime and does not significantly decrease the LED brightness.

You can start from a 100-Ohm resistor and increase it and simultaneously monitor the current. In my case, I read 15mA by using a 180-ohm resistor.



Figure 2

Finding the best resistor value for the series LEDs


References

Source: https://www.pcbway.com/blog/technology/Programmable_Police_LED_Flasher_Using_an_STM8.html

[1]: https://www.st.com/resource/en/datasheet/l78m.pdf

[2]: http://www.ti.com/lit/ds/symlink/lm1117.pdf

[3]: https://www.st.com/resource/en/datasheet/stm8s001j3.pdf

[4]: http://www.vishay.com/docs/91192/sihfl110.pdf

[5]: https://componentsearchengine.com/part.php?partID=1049204

[6]: https://componentsearchengine.com/part.php?partID=4735

[7]: https://componentsearchengine.com/part.php?partID=4735

[8]: https://componentsearchengine.com/part.php?partID=179508

[9]: https://componentsearchengine.com/part.php?partID=235673

[10]: https://www.samacsys.com/altium-designer-library-instructions

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