Timer Board

Project Overview

Boulder, CO (September 2024) - This board was designed to practice the full Printed Circuit Board (PCB) design process. The board uses a 555 timer to drive four light-emitting diodes (LEDs) with various current limiting resistors in a configuration typically seen for indicator LEDs. There is a tradeoff between indicator LEDs being bright enough to convey their meaning and the additional power consumption required for the part. This demonstration will show the brightness of red LEDs being driven with different amounts of current which can be used as a reference for future board designs. Ultimately, a current of 1mA for the red LEDs was optimal regarding the balance between brightness and power consumption. This was demonstrated by the LEDs with 1K current limiting resistors.

Figure 1 shows the proposed design for the PCB. The 555 timer will be set up to drive the output at approximately a 50% duty cycle at 500Hz. The specific values for the output do not need to be accurate, priority will instead be given to the simplicity of the design and to using common parts if possible. The LED circuit will compare four brightness levels by using current limiting resistors ranging from 50 to 10K Ohms.

Acceptance Criteria

1. The power rail is stable at 5V.

2. The 555 timer output duty cycle is between 40% and 80%.

3. The 555 timer output duty cycle frequency is between 200 and 1,000Hz

4. LEDs are powered and there is no visible flickering.

5. The current through one of the LEDs can be measured.

Figure 1: Initial sketch of the proposed design. The board is comprised of three major functional groups: power conditioning, the timer, and the LED array. The board will receive power from a 5V source driving a 555 timer which will, in turn, directly drive four LEDs. Switches and test points will be incorporated to isolate and analyze the functional groups.

Figure 2: The schematic for the design. The 555 timer uses two 1K Ohm resistors and a 1 μF capacitor. The current limiting resistors selected were 10K, 1K, 300, and 47 Ohms.

Board Design Features

Figure 2 shows the schematic for the board. The power conditioning functional group is powered by a barrel jack and includes an LED to indicate when power is applied to the board, a switch to isolate power from the rest of the circuit, and a test point to verify the power rail voltage. The timer functional group uses two 1K resistors and a 1 μF capacitor for the NE555P timer. These values were based on the results of a 555 timer comparison experiment1. Finally, the LED functional group is isolated from the 555 timer by a two-way switch and uses a 10K, 1K, 300, and 47 Ohm resistor to limit the current through the LEDs. A test point on the 1K resistor allows for direct current measurement through one LED.

Figure 3: The final board layout showing the reference plane on the back of the board in blue, the top layer copper traces in red, the bottom layer copper traces in black, and the text on top of the silk screen in yellow.

Figure 4: The board received from the manufacturer matches the board layout seen in Figure 3 building confidence that the manufacturing process was done correctly.

Figure 5: The fully assembled board with 5V power suppled. All LEDs light up with expected relative brightnesses based on their respective current limiting resistors which are indicated on the board. The 5V power indicator LED uses a 1K resistor.

Figure 3 shows the final board layout which closely matches the schematic. Figure 4 shows the bare board. Figure 5 shows the fully assembled board with power applied. The initial bring-up of the board was qualitatively successful based on the LED array lighting up with expected relative brightness levels for each current limiting resistor. Additionally, and as expected due to the duty cycle driving the LED array, the power indicator LED, which has a 1K current limiting resistor, was slightly brighter than the 555 timer-driven LED with the same size current limiting resistor.

Figure 6: The output from TP1 showing a stable 5V power rail when the isolated from the rest of the circuit.

Table 1: Estimated current through resistors in the LED array. The Current column is an estimate of the current through the LEDs while the signal is high. The Average Current column accounts for the 70% duty cycle of the 555 timer output.

Next, the board was evaluated for expected functionality. The 5V power rail was measured from TP1 to show that the board was receiving the appropriate power (Figure 6). Figure 7 shows the rise time of the output from the 555 timer with the LEDs disabled. Comparing this to Figure 8, where the LEDs are enabled, shows having the LEDs on increases the rise time slightly and causes a 160mV drop in the steady-state voltage. Figure 9 and Figure 10 compare the output of the 555 timer when the LEDs are disabled and enabled, respectively. The duty cycle and frequency of the output remain stable across the two cases, however, a drop in the signal voltage is observed when the LEDs are activated, which was noted when reviewing the rise times in Figure 7 and Figure 8. Figure 11 shows the voltage drop over the 1K current limiting resistor in the LED array to be 1.63 volts.

Figure 7: Shows the peak-to-peak voltage, rise time, and overshoot of the output of the 555 timer circuit with the LEDs disabled. Based on these values, the top of the signal can be calculated as 5.1V / 1.337 = 3.8V.

Figure 8: Shows the peak-to-peak voltage, the rise time, and the overshoot of the 555 timer output with the LEDs enabled. Based on these values, the top of the signal can be calculated as 4.6V / 1.261 = 3.6V.

Combining the output voltage from the 555 timer when the LEDs are enabled (Figure 8) with the voltage drop over the 1K resistor (Figure 11) gives a voltage drop over the LED of 3.6V 1.6V = 2.0V, which aligns with the expected voltage drop over a red LED and builds confidence that the measurements are correct. Using the 1.63 volt drop over the 1K resistor, the current through the associated LED is I = V/R = 1.63V/1K = 1.6mA. Using similar calculations, the total estimated current for the LEDs is summarized in Table 1. Using the total current from the LED array with the 160mV drop in the output signal from the 555 timer when enabling the LEDs, gives the estimated Thevenin voltage of the 555 timer from R = V/I = 160mV/41.8mA = 4Ω.

Figure 9: Shows the duty cycle and frequency of the output of the 555 timer with the LEDs disabled.

Figure 10: Shows the duty cycle and frequency of the output of the 555 timer with the LEDs enabled.

Figure 11: Shows the voltage drop over the 1K current limiting resistor in the LED array.

Finally, the switching noise on the 5V rail was measured to ensure the board design was properly mitigating large variations in voltage (Figure 12). The voltage drop is perceptible but small in relation to the noise on the rail. This indicates that the practices used did a good job mitigating switching noise when compared to previous decoupling capacitor experiments [2].

Figure 12: Shows the switching noise on the 5V rail (depicted in green) triggered on the output of the 555 timer (depicted in yellow) is on the order of tens of millivolts.

Lessons Learned

The results of this experiment indicate that 1mA is sufficient current for the red LEDs to be easily visible while limiting the amount of power consumed. This will be incorporated into future red indicator LEDs. The spacing of components worked well for assembly; in future designs, components can be brought slightly closer together to create a more compact design. Additionally, the test points, isolation switches, and indicator LEDs were extremely helpful in quickly determining that the board was functioning as intended. These features will continue to be used in future designs. During assembly solder paste and a heat gun were used. This process made placing the components much easier leading to the individual components being visible aligned well and to the assembly taking less time.

Fortunately, there were no hard errors during the bring-up and testing of this board. This is in part because this was a practice board for which the design was provided. However, one soft error is that the measured frequency does not match the expected frequency for the 555 timer (320Hz instead of closer to 500Hz). The value is still within the acceptance criteria, but if substantially different than what was found during initial testing1. The reason for this substantial difference is unclear since the components were verified to be correct. The difference may be due to manufacturing flaws in the components used. If time permits, it would be worth assembling a second board to see if the results are the same and then continue troubleshooting from there.

There were a few key takeaways from the routing of traces. Two cross-unders were used for the 555 timer traces; upon closer inspection, these were unnecessary had the traces been routed slightly differently. When using a 555 timer in the future, cross-unders will be avoided. Additionally, having traces run through text makes it slightly harder to read the text so that practice will be avoided in the future. Finally, some traces had unnecessary branching of signals; direct lines should be used when possible.

References

[1] Center, J. D. (2024 August). 555 Timer Comparison. Jack’s Portfolio.
https://www.jackdcenter.com/pcb-design/555-timer-comparison

[2] Center, J. D. (2024 September). Decoupling Capacitors. Jack’s Portfolio.
https://www.jackdcenter.com/pcb-design/decoupling-capacitors

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