Two versions of soft start

UPDATE: 08.02.2019
There is upgraded version of this project: Push button ON/OFF + Soft start v.3
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 My new project is something that is useful when creating power amplifier with toroidal transformer. Usually the starting (inrush) current is very high for a brief moment until the smoothing capacitors are charged. This is stressful for the capacitors, bridge rectifiers and the transformer itself. Also the fuse may blow.
The soft start circuit is used to limit the starting current to acceptable level. This is achieved by connecting the transformer to the mains via a resistor which is shorted with relay contacts a few moments later.

I decided to combine a soft start schematic with push button toggle switch schematic to create a module ready to be used in a power amplifier or some other power consumer.

I experimented with two different schematics - one with CMOS logic 4027 and other with dual timer NE556.

Advanced LC meter

After finishing my last project - "Simple LC meter", there were some discussions in the forum I am a member of, that ability to measure electrolytic capacitors would be very useful in this type of device.
I searched the Web and found a very cute project named LCM3 on this Hungarian site: hobbielektronika.hu . I love Hungarian rock since my school days, but I don't know a word in Hungarian :( . So, I searched the Web again, this time for this specific project and found a Russian forum where the project was discussed in details and I got more useful information about parts, settings and so on.

Specifications of the LCM3 are (according to authors of the project):
Capacitors:
from 1pF to 1nF - resolution: 0.1pF, accuracy: 1%
from 1nF to 100nF - resolution: 1pF, accuracy: 1% 
from 100nF to 1uF - resolution 1nF, accuracy: 2.5%
Electrolytic capacitors:
from 100nF to 100 000uF - resolution 1nF, accuracy: 5%
Inductance:
from 10nH to 20H - resolution 10nH, accuracy: 5%
Resistance:
from 1mOhm to 0.5Ohm - resolution 1mOhm, accuracy: 5%

Simple LC meter

Here is another piece of laboratory equipment - LC meter. This type of meter, especially L meter is hard to find in cheap commercial multimeters. Schematic of this one came from this web page: https://sites.google.com/site/vk3bhr/home/index2-html. It uses PIC microcontroller 16F628A, and because I recently acquired a PIC programmer, I decided to test it with this project.

Audio Oscillator with Frequency Counter

I am continuing to add DIY equipment to my home workshop. Audio oscillator is a very useful tool that is needed when we test audio projects. I already have a similar tool but it is just an oscillator without a frequency counter. I decided to use the schematic described in this page:  http://www.redcircuits.com/Page82.htm.
It is a standard Wien Bridge oscillator but uses a very interesting way to stabilize amplitude - optocouple of photoresistor and LED.

Power Amplifier with TDA2050

This is not an ordinary project, but an attempt to make a PCB that is suitable for TDA2050 and LM1875 and has all the necessary circuitry on board - power supply, speaker protection,  delayed turn-on and fast turn-off. This is achieved using the convenient uPC1237 IC.

Speaker protection with uPC1237

I was using IC uPC1237 for speaker protection in almost all my power amplifiers, because schematic is very easy to build with few external components.