Summary: The objective of this project was to create a visual signal on my treadmill that would alert me when someone was ringing the front doorbell. Why a visual alert? Well, because I invariably have music blaring out of my in-ear headphones when I’m running and 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 that the doorbell had been rung. The final design involved a large red flashing LED to get my attention, controlled by a 556 timer chip (which is two 555 timer circuits in one package) and wired through internal house wiring to the doorbell. The enclosure housing the device was designed to fit into one of the bottle holders on the treadmill display console.


The objective of this project was to create a visual alert light which would become activated when someone was ringing the front doorbell.


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 necessary aid when running so 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 some sort of visual indicator device was needed on the treadmill that would alert me to when someone was at the front door.

Photograph of the Smooth Fitness Evo 3I treadmill

Obviously, I didn't want to be staring directly at a device the whole time I was running just in case someone rings the doorbell, so 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!). Therefore, the visual alert device needed to be a flashing light that was triggered by the doorbell.

Since the doorbell is powered off low DC voltage, it made sense to use the same power to run the alert device as well. However, my doorbell at the moment is running off a 9V battery, so I didn't want the flashing alert light to be on indefinitely when triggered as it would end up using up battery power unnecessarily, and I certainly didn't want the device itself to be powered when I was not on the treadmill. Therefore, the device had to first have some sort of timing mechanism where the flashing light would go off after a while (approximately 60 seconds as my delivery guy doesn't wait much longer than that!), and secondly, it also needed to have a power switch to turn it off when not in use. Finally, it would also be useful for it to have some sort of indication of when it was powered up instead of relying solely on observing the position of the switch.


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, the integrated circuit that beginner electronics students are often first introduced to when they begin their electronics education. A 555 timer was needed to control the the frequency of the flashing light itself, while another timer was used to control the length of time the flashing is 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, an astable 555 circuit was used to control the flashing light's tempo, while another 555 timer was configured as a monostable circuit that controls the length of time the flashing light is active.  Since two individual 555 timers were needed for the device to operate, it made more sense to use a 'dual 555' timer chip, designated the 556 Timer, which 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 that sends the doorbell magnetised hammer shooting into an initial chime bar to create a 'ding' sound, while at the same time compressing 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 where it hits a second chime bar completing the typical 'ding-dong' sound that is characteristic of basic doorbells.

Power for the doorbell alert device is derived from the 9V battery of the bell itself so connections were made to V+ (white & orange wire), V- (blue wire), and the trigger for the monostable circuit (white and blue wire) - Figure 2.

The Circuit

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

Figure 3. The schematic

Each 555 timer circuit on the dual 556 IC  is configured into either a monostable configuration or an astable one (see Figure 1). The trigger line for the monostable circuit is connected to the doorbell so that when the doorbell circuit is complete (ie. the doorbell is pressed), the trigger line is momentarily pulled to ground, initiating activity on the monostable circuit timer. The output from this first 555 circuit is connected 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, which 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 alert light.

Making the PCB

The circuit board was made using the PCB photo-etching technique (Figure 5) using a single-sided photo-sensitive copper-clad board. The PCB had to fit inside a cup holder on the treadmill (see 'Enclosure' below) which 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, and for PCB mounting holes, a standard pillar drill was used 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 the 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 alert device,  I finally settled on the least destructive option which was to use 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.

The enclosure for the alert device 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 had to incorporate 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, wires were soldered onto their respective terminals and insulated in heat-shrink tubing. The wires ended up being a little longer than planned but since I had ample room in the treadmill cup to accommodate the longer wires, this was not to be a problem. At the other end of the connecting wires, female open-barrel pins were crimped onto the bare wires and the crimp pins were placed into appropriate crimp housings, allowing the LEDs and the switch to be plugged into the appropriate header pins on the circuit board.

For the mounting of the circuit board itself to the enclosure, instead of using separate PCB spacers for PCB mounting, the mounting holes were modelled right into the design of the enclosure so that they were raised above the bottom of the enclosure and 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 on special on Amazon so don't be surprised if more pink projects make their way onto this website! 🙂)


A hole was drilled in the bottom of the treadmill bottle holder cup and an ethernet patch cable was re-purposed to connect the device (once again using pin headers on the circuit board and female crimp pins on the cable) to an ethernet wall socket whose wiring had been redirected to terminate at the doorbell box (Figure 2).

The Finished Project

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