Summary: The objective of this project was to create a visual signal on my treadmill to alert me when someone was ringing the doorbell. Why a visual alert? Well, because I invariably have music blaring out of my in-ear headphones when I’m running. Between that and the noisiness of the treadmill itself, I have, in the past, missed the delivery guy knocking at the front door. It is so annoying to have to go on a treasure hunt just to recover parcels that I could have easily received myself if only I had known someone was at the front door. The final device I settled on consisted of a large red flashing LED to get my attention, controlled by a 556 timer chip (which is two 555 timer circuits in one package). I then connected it through internal house wiring to the doorbell. In addition, I designed the enclosure housing this doorbell light to fit into one of the bottle holders on my treadmill's display console.


The objective of this project was to create a visual alert light that activates when someone rings the front doorbell.


Missing Deliveries

I am an avid runner and I often use a treadmill at home. However, as I'm sure most runners would agree, music is a essential aid when running. Therefore, putting on a pair of headphones is an essential part of my running routine. Unfortunately, between the music in my ears and the noise of the treadmill itself, it is pretty easy to miss the sound of the doorbell. In this day and age, when shopping online and home deliveries are more common than going out to the shops, and when missing the delivery guy means a whole lot of chasing around to find one's package, I decided that I needed some sort of visual indicator on my treadmill to alert me when someone was at the front door.

Flashing Doorbell Light

Photograph of the Smooth Fitness Evo 3I treadmill

Obviously, I didn't want to be staring directly at a device the whole time just in case someone rings the doorbell. Therefore, some sort of flashing light was the obvious answer as human peripheral vision has evolved to be very good at detecting visual changes in the environment (being able to detect a predator jumping out of the bushes beside you is a pretty good evolutionary pressure!). So, I needed the visual alert device to be a flashing light that was triggered by the doorbell.

Most doorbells get their power from low DC voltage. So, it made sense to use the same power to run the doorbell light as well. However, my doorbell is running off a 9V battery. Therefore, I didn't want the flashing light to be on indefinitely as it would use battery power unnecessarily. Furthermore, I certainly didn't want the device to be powered when I was not on the treadmill. Therefore, the device had to have some sort of timing mechanism where the flashing light would stop after a while (approximately 60 seconds as my delivery guy doesn't wait much longer than that!). It also needed to have a power switch to turn it off when not in use. Finally, it needed some way to indicate when it was powered up to that I wouldn't leave it on unintentionally.


555 Timer

Sketch of the component connection schematic for the doorbell alert light device

Figure 1. Planning

One of the simplest ways to generate a timing circuit is to use a 555 timer. This integrated circuit is often one of the first introduced to beginner electronics students. I needed a 555 timer to control the the frequency of the flashing light itself. Another timer would control the length of time the flashing was active for. The great thing about the 555 timer is that it can be configured to function in different ways: astable, monostable and bistable. In this application, I used an astable 555 circuit to control the flashing light's tempo. Another 555 timer was configured as a monostable circuit. This controls the length of time the flashing light is active for. Since I needed two individual 555 timers for the device to operate, it made more sense to use a 'dual 555' timer chip, designated the 556 Timer. This is essentially two 555 timers in one IC package.

The Doorbell

The interior of the battery-operated doorbell wired up to the doorbell alert light

Figure 2. The doorbell wired up

My doorbell is a pretty simple circuit consisting of a button, electromagnet and 9V battery (Figure 2). Essentially, pressing the doorbell completes a simple circuit, activating the electromagnet. This sends the doorbell's magnetised hammer shooting into an initial chime bar to create a 'ding' sound. At the same time, the hammer compresses a spring. When the doorbell button is released, the magnetised hammer is sent back in the opposite direction by the energy stored in the compressed spring. It then hits a second chime bar completing the typical 'ding-dong' sound that is characteristic of basic doorbells.

The doorbell light derives its power from the 9V battery of the bell itself. Therefore, I made connections to V+ (white & orange wire), V- (blue wire) to power the device (Figure 2). In addition, the connection to the trigger (Trig; white and blue wire) was for the monostable side of the circuit.



The Circuit

Schematic of the printed circuit board for the doorbell alert light device

Figure 3. The schematic

I configured each 555 timer circuit of the dual 556 IC to either a monostable configuration or an astable one (see Figure 1). The trigger line for the monostable circuit connects to the doorbell. Therefore, when the doorbell circuit is complete (ie. the doorbell is pressed), the trigger line is momentarily pulled to ground. This initiates activity on the monostable circuit timer. The output from this first 555 circuit connects to the Reset pin of the 2nd 555 (astable) circuit on the other side of the IC. While the first 555 monostable circuit is counting down its timer, it holds its output line high. This, in turn, holds the astable reset line high and thus holds the astable circuit in active mode, allowing it to do its job of flashing the doorbell light.

Making the PCB

I made the circuit board using a PCB photo-etching technique (Figure 5) with a single-sided photosensitive copper-clad board. The PCB had to fit inside a cup holder on the treadmill (see 'Enclosure' below). This limited it to a size of approximately 45mm x 45mm (Figure 4.) The holes for electronic components were drilled with a 1mm drill bit on a Proxxon mini-drill and drill-stand. For the PCB mounting holes, I used a standard pillar drill as a 4mm bit did not fit in the Proxxon Drill. In retrospect, deciding on mounting screws that were so thick was a bad idea since my Pillar drill is far less accurate and harder to control than the Proxxon Drill (Note-to-self:  future circuit boards should use smaller (maximum) 3mm screws for mounting).

Board layout of the printed circuit board for the doorbell alert light device

Figure 4. PCB layout

Etching artwork of the printed circuit board for the doorbell alert light device

Figure 5. Toner-transfer artwork

The photo-etched printed circuit board for the doorbell alert light device

Figure 6. Etched PCB

The Enclosure

The device had to sit nicely and securely on a Smooth Fitness Evo 3i treadmill. After much debate on how to modify the treadmill to accommodate the doorbell light, I finally settled on the least destructive option. This involved using one of the 'bottle-holding' cups that were a design feature of the running machine (see Figure 7).

The bottle holder on the Smooth Fitness Evo 3I treadmill

Figure 7. Bottle holder on the treadmill

This was especially appropriate since I had never actually used them for their intended purpose of holding drinking bottles.

3D printing

The enclosure for the doorbell light was modelled in the open-source 3D-modelling software, Blender, and printed out on a Velleman K8200 3D printer (Figure 8).

The completed doorbell alert light device 3D modelled in Blender

Figure 8a. 3D model of the enclosure

The 3D-printed enclosure for the doorbell alert light device

Figure 8b. The 3D-printed enclosure

The enclosure incorporated a large red 'alert' LED in the middle and a small green 'power-on' LED to one side, as well as the on-off switch. In addition, for these two LEDs and the power switch, I soldered wires onto their respective terminals and insulated them with heat-shrink tubing. The wires ended up being a little longer than planned but since I had ample room in the treadmill cup, this was not a problem. At the other end of the connecting wires, I crimped on female open-barrel pins onto the bare wires with the crimp pins placed into appropriate crimp housings. This allowed me to plug in the LEDs and the switch into the appropriate header pins on the circuit board.

PCB Mounting

For the mounting of the circuit board itself to the enclosure, I did not use separate PCB spacers for PCB mounting. Instead, I modelled the mounting holes right into the design of the enclosure so that they were raised above the bottom of it. These were ready to accept four 6 x 1/2" panhead screws (Figure 9a & 9b).

The assembled PCB and 3D-printed enclosure for the doorbell alert light device (bottom view)

Figure 9a. The finished PCB & enclosure

The assembled PCB and 3D-printed enclosure for the doorbell alert light device (side view)

Figure 9b. The finished PCB & enclosure

(NB: As with other recent projects, the enclosure is pink in colour, not by preference, but due to economics. Pink filament was selling at a discount on Amazon so don't be surprised if more pink projects make their way onto this website! 🙂) 


I drilled a hole in the bottom of the treadmill bottle holder cup and re-purposed an ethernet patch cable to connect the device to an ethernet wall socket. I had redirected the socket's wiring to terminate at the doorbell box (Figure 2). Once again, I used pin headers on the circuit board and female crimp pins on the severed end of the ethernet cable for easy plug-in connections.

The Finished Doorbell Light

The doorbell alert light installed on the Smooth Fitness Evo 3I treadmill

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Materials Used

TypeComponent Quantity
Resistors82 Ω82Rsmall
1 kΩ 1K
4.7 kΩ4.7K
10 kΩ10K
270 kΩ270K
1 MΩ1M
1 µFCPOL_1µF
4.7 µFCPOL_4.7µF
47 µFCPOL_47µF
100 µFCPOL_100µF
10 nFCeramicCap_10nF
100 nFCeramicCap_100nF
556 TimerNE226NNE556N
Header2 & 3 pins (straight)4-pinHeader
IC Socket14-wayIC_connector_14-way
Photo Etch
LEDs3mm Green LED3mm_Green_LED
20mm Red LED20mm_Red_LED
LED Holders3mm LED Holder3mm_LED_holder
20mm LED Holder20mm_Red_LED_holder
Switchsingle pole single throw (SPST) small_rocker_switch