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SainSmart Frequency Meter

Thanks to Lewis, M3HHY for lending me this one 🙂

Here’s a quick look at a Sainsmart frequency counter module. These are useful little gadgets, showing the locked frequency on a small LCD display.

It’s built around an ATMega328 microcontroller (µC), and an MB501L Prescaler IC. The circuit for this is very simple, and is easily traced out from the board.

Frequency Counter
Frequency Counter

Here’s the back of the board, with the µC on the left & the prescaler IC on the right. This uses a rather novel method for calibration, which is the trimmer capacitor next to the crystal. This trimmer varies the frequency of the µC’s oscillator, affecting the calibration.

Input protection is provided by a pair of 1N4148 diodes in inverse parallel. These will clamp the input to +/-1v.
The prescaler IC is set to 1/64 divide ratio. This means that for an input frequency of 433MHz, it will output a frequency of 6.765625MHz to the µC.

The software in the µC will then calculate the input frequency from this intermediate frequency. This is done because the ATMega controllers aren’t very cabable of measuring such high frequencies.

The calculated frequency is then displayed on the LCD. This is a standard HD44780 display module.

LCD
LCD

Power is provided by a 9v PP3 battery, which is then regulated down by a standard LM7805 linear regulator.

Readout
Readout

I’ve found it’s not very accurate at all at the lower frequencies, when I fed it 40MHz from a signal generator it displayed a frequency of around 74MHz. This is probably due to the prescaler & the software not being configured for such a low input. In the case for 40MHz input the scaled frequency would have been 625kHz.

 

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Tool-less UK Mains Plug

Plug Bottom
Plug Bottom

This is a standard 13A UK main power plug – with a twist. This one requires no tools to open or connect.
The top cover is slid off the top, after turning the red cam with a coin to unlock it.

Plug Top
Plug Top

Manufactured by Plugco – a Google search of this company returns no results.

Cover Removed
Cover Removed

Top cover removed from the plug, as is standard with UK mains plugs the live conductor is fused – 3A in this case. The conductors are clamped into the fittings in a row along the top edge of the plug.

Wire Clamps
Wire Clamps

Closeup of the wire clamps. Conductor is placed in the slot & snapped closed.

This is an old plug & they do not appear to be available these days, for unknown reasons. Being able to change a plug without a screwdriver has it’s advantages 🙂

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PSi 150 Power Inverter

Front
Front

This is a small 120W power inverter, intended for small loads such as lights, fans, small TVs & laptop computers.

End Cover
End Cover

End cover of the unit, 12v DC input cord at the top, power switch & indicator LEDs at the bottom.

Mains Output
Mains Output

Opposite end of the unit, with the standard 240v AC 50Hz Mains output socket.

Cover Removed
Cover Removed

Cover removed from the top of the unit. Main power transformer is visible in the centre here, MOSFET bank is under the steel clamp on the left, the aluminium case forms the heatsink.

PWM Controllers
PWM Controllers

On the right is a KA3525 switchmode PWM controller & on the left is a LM324N quad Op-Amp IC. The buzzer on the far left is for the low battery warning.

PCB Removed
PCB Removed

PCB removed from the casing, with the MOSFET bank on the right hand side. Two potentiometers in the centre of the board tweak the frequency of the switcher & the output voltage.