Tachostrobe

A friend came to me whose vinyl turntable didn't have a stroboscope, which is why he make a printed stroboscope disc. And he attached a neon light as illuminator. And he came to me with a great idea - the frequency of the mains is not precise, why not make something on a LED with quartz stabilization? In principle, there are probably a lot of similar devices from China on AliExpress, but he really didn't want to wait until it arrived from there.

Than I took a piece of PCB from one of my old device, where was the place for small microcontroller (STM32C011) and in half an hour the “vinyl flashlight” was made.

The next day, I thought - why only a stroboscope? Let make also a tachometer. As a result, this device was created, 2 in 1 "Tachostrob".

I'm sure something similar can be bought on AliExpress, but someone might want to make it with their own hands, just with some parts from Ali or other cheap source. The most expensive is the battery - $ 4.3, and the processor and photo sensor - about $ 2 retail.

Now the description of the device:

- A small board, 66x30mm, all the parts on top, only the battery on the bottom.

- On the left is a mini USB connector for charging, next to it there are two pads, you can supply 5V there. Battery 601613 for 300mAh (“Chinese mAH”), and of course, you can put any other, suitable in size. Current consumption is around 30mA, and as the operating time is short, the battery will work a long time. For indication during charging there are two LEDs, at the top left. The red one lights up during charging, the green one - at the end.

- In the center there is an OLED display, and below there are connectors for initial programming and two control buttons.

- On top, in addition to the charger LEDs, there is also a tachometer indicator LED and a network frequency jumper (50/60 Hz)

- On the right there is a strobe LED and a tachometer photo sensor.

- The strobe pulses are quite short (duty cycle is 5%), so it is better to use the brightest LED as possible.

- For the tachometer to work, a reflective (if the player's disc is dark) or light-absorbing (if the disc is shiny) mark is required. I used a 1.5-2 cm piece of self-adhesive aluminum foil glued to the side surface of the disc, but you can also stick it on top. A piece of black electrical tape can be used as a light-absorbing mark.

- The controls are very simple – press (and hold) one button, the strobe works, press another – the tachometer works.

- In the strobe mode, shine the light on the disk with the marks. The display indicates the mode and network frequency (50/60 Hz), set by the jumper J8. The LED flashes at a frequency of 100 or 120 Hz.

- In the tachometer mode, shine the light where the reflective/absorbing mark is glued. Before the tachometer works, you need to select the distance from the sensor to the mark. To do this, place the sensor opposite the mark, rotate the disk by hand and achieve the operation of the tachometer indicator LED. We select a distance so that the LED lights up and goes out confidently when passing the mark.

- The tachometer measures the disk rotation frequency from 10 to 100 RPM. At the same time, if it is into the range near the real values of turntable’s standard RPMs (16, 33 1/3, 45 and 78) the average deviation in percent for 3 revolutions is also displayed, with an accuracy of 0.01%.

The diagram, board gerbers and firmware hex-file are free. You can flash via SWD (STLink) via connector J 6, or via connector J 7 using USB2COM (or any other UART ) with the STM32CubeProgrammer software (fee from ST web site) . When using UART , it is advisable to set the nBOOT 1 bit in Option Bytes – this will allow you to reflash the chip again in the future, when installing jumper J 5. When flashing a new clean chip, you do not need to install this jumper.

0.91” OLED display usually have 3.3v regulator on board and place for resistor R5, which can be short, because display is powered from 3.3v here.