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Have you ever wished your system could tell you when the utility supply comes back on? Suppose you’re running a bench power supply for projects and you want not just 5V output, but also a little buzzer or alert when the mains utility returns. In this article, I’ll walk you through how to design a linear power supply that delivers 5V DC and integrates a utility‐alert circuit—a monostable that triggers a buzzer for about five seconds when the utility supply is restored.
Remember you’ve already built a 5 V/12 V/18 V linear supply as covered in your other post (Click Here To View Post), you’re in a great place to extend that to this design. We’ll skip the schematic diagram (since you already have one working) and focus on the practical walk‐through of the alert integration, design considerations, breadboarding, soldering, testing, casing and tips. Let’s dive in.
When you build a linear power supply, you get quieter (less ripple, less switching noise) and more predictable output—ideal for analog circuits, sensors, audio and similar gear. The linear regulator simply drops excess voltage rather than chopping it. (Wikipedia)
In many places, utility power may go off and come back unpredictably. A little alert that “Hey, power is back!” is useful: your circuits can resume, you can manually check things, or simply know it’s safe. Rather than relying on indicator lights or watching mains, your device signals audibly for about five seconds.
This kind of project teaches you not only power supply design but also how to build monostable circuits using discrete transistors (here, TIP41C NPNs), resistors and capacitors. That’s a good learning experience.
Since you already have the 5V linear power supply part, here’s what you add for the alert system:
A monostable device is a circuit that, when triggered, changes state for a predetermined period then returns to its original state. In our case, when utility power returns, we trigger the monostable and the buzzer sounds for approx. 5 seconds.
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Using the two TIP41C transistors: one transistor senses the incoming 5V presence (or the AC detection), and another drives the buzzer. The timing capacitor and resistor set the 5 s window. The advantage: simple, no microcontroller, easily built on a breadboard.
To get around 5 seconds, you pick a resistor-capacitor pair, for example ~10kΩ and ~470 µF, then the transistors shape the pulse. You’ll want to test and adjust the values for accurate time.
You can detect the return of utility supply either by sensing the AC via a small transformer or by sensing the 5V DC becoming present again. When the 5V rail comes alive, you trigger the monostable circuit. That’s a neat trick: the supply itself becomes the detector.
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Since the buzzer is powered by the 5V rail (same as your supply), the transistor switch drives the buzzer for the set time. When the monostable times out, the buzzer turns off, leaving your main 5V rail unaffected.
Mount the alert circuit on the same board or a daughter board near the 5V output section. Ensure you keep signal wiring short for detection transistor, and keep load wiring separate to reduce interference.
Tap the 5V regulated output as the supply for the alert circuit. The detection transistor sees the presence of the 5V rail to trigger. Use a clean ground common between supply and alert circuit.
Since the buzzer is a noisy load (acoustically and perhaps electrically), consider adding a small decoupling capacitor on the 5V rail feeding the alert circuit to avoid injecting noise back into your load.
Even though it’s a linear supply, you still need to ensure heat dissipation and proper insulation. The alert circuit components may get warm, but usually modest. Provide ventilation and avoid excessive heat build-up.
Lay out components logically: transformer side, rectifier side, filter capacitors, regulator, then alert circuit. Keep traces short for high-current paths, and keep signal traces (for alert circuit) separate.
After soldering, visually inspect for cold joints, solder bridges, especially around the alert circuit where timing may be sensitive. Use a magnifier if needed.
Mount the buzzer securely in the case (or externally if desired), and add screw terminals or USB connectors (if you plan to power USB loads) for your 5V output. Although this design is 5V only, you might have other rails in your previous design.
Power on the supply without load. Measure the 5V output; check ripple, noise, and stability. Measures with a multimeter or oscilloscope if available.
Connect a modest load (for example; a 12 V fan) and verify voltage remains stable. Check the alert circuit doesn’t falsely trigger under load.
Simulate utility supply loss and return: turn off the transformer input or disconnect mains briefly, then restore. When input is restored, the 5V rail comes up; the alert circuit should trigger the buzzer for ~5 seconds, then stop.
Leave the system on for some time under load and check temperature of regulator, transformer, alert circuit components, and verify the buzzer timing remains consistent.
Check for overheating, check that there is no hum, ensure earth/ground connections are safe, and if this will be used long term, consider adding fuse or protection.
Choose a project box that can safely accommodate the transformer, PCB, connectors and buzzer. Metal box is good for shielding; plastic is easier but may need ventilation.
Secure the transformer to the box base, mount the PCB with standoffs, mount the buzzer in a location where sound is audible (maybe a grill). Add ventilation holes near the regulator / transformer.
Use cable ties or adhesive anchor points to keep wiring tidy. Keep AC/mains and DC/output wiring separated to reduce noise and interference.
Mark input voltage ratings, ground symbol, caution “230 V inside” if mains is high, ensure the enclosure is safe for your region’s mains standard.
Check the detection transistor: is it seeing the 5V rail when it comes up? Measure base voltage when the rail comes alive.
Check the timing resistor/capacitor: are values correct? Are they connected properly?
Adjust the timing resistor or capacitor. If too long, reduce R or C; if too short, increase them. Ensure capacitor is within spec.
Your linear regulator or transformer may be undersized. Try a lower load, check for overheating, check ripple, ensure filter capacitors are adequate.
Check wiring for loops, keep AC and DC wiring separated. Add decoupling capacitors. The alert circuit’s switching may introduce noise—filter it if necessary.
Check ventilation. Possibly use a metal case with heat sink or add ventilation holes. Ensure transformer and regulator aren’t overloaded.
Yes—it will trigger when the 5V rail returns. If the mains is offline for a long time, the circuit simply stays idle until there’s power again. Just ensure the timing capacitor hasn’t leaked or degraded.
You change the resistor and/or capacitor in the timing network. For example doubling the capacitor value or resistor value roughly doubles the time constant, so you’d go from ~5 s to ~10 s.
Yes—any NPN transistor with appropriate voltage (≥ 5V) and current ratings will do. Just check the buzzer current and ensure the transistor can handle it.
No. The beauty of this design is that it uses a simple discrete monostable circuit with transistors and RC timing. It keeps things low-cost and easy to build.
If the 5V rail dips or disappears and then returns quickly, the alert circuit may trigger repeatedly or not at all if timing overlaps. You can mitigate this by adding a small delay (via a capacitor) or hysteresis so brief flickers don’t cause unwanted alerts.
Yes—you can invert the logic. Detect mains presence continuously; when it disappears, trigger the monostable and buzzer. That may require a different detection arrangement (e.g., a transistor held on when supply is present, and goes off when it disappears).
No—not much. When idle (no trigger), the transistors are off, and the only draw is from bias resistors etc., which is minimal compared to the supply load.
Yes—just ensure you change component values and the regulator to match the target voltage, and adjust the alert circuit supply voltage accordingly.
Generally yes—if your regulator and transformer are sized to handle the extra current. The alert current is brief and small, but you should still design for worst-case load plus alert.
Linear supplies are less efficient (waste more heat) compared to switch-mode. They may require bigger transformers and heat sinks. But for clean output and simple integration with your alert circuit, they’re very acceptable.
Designing a linear power supply with a utility alert system is a practical and rewarding project. You get the best of both worlds: a clean, regulated 5V output for your electronics plus a built-in audible alert when the mains utility returns. It’s more than just a supply—it’s a smart little companion to your bench or project environment.
By integrating a simple monostable circuit with discrete transistors and RC timing, you’ve kept things straightforward yet effective. You’ve sharpened your skills in power supply design, timing circuits, breadboarding, soldering, testing, and finally casing a project.
Whether you’re building this for yourself, for a lab, or as a teaching exercise, the result is functional, useful, and elegant. So grab your parts, build it up, test it thoroughly—and enjoy the satisfaction of hearing that buzzer when the power comes back on, knowing you built it.
Happy building!
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