This is an 125 kHz RFID reader that is based on ATtiny13 micro-controller and an LM358 Operational Amplifier. No special RFID chip is used. The reading, decoding and printing the unique ID from 125 kHz RFID tags is made entirely in software by ATtiny13.
125 kHz RFID reader based on ATtiny13 - [Link]
The circuit uses both op-amps of an LM358 to control the charging of a single cell lithium ion battery. Charging automatically stops when the battery is full, and it is possible to charge batteries that have gone below the undervoltage limit. Power is provided through USB or any other 5V source.
Simple USB DIY Li-ion battery charger - [Link]
When building AVR DDS2 signal generator there were lots of discussions about signal conditioning in analog part of device. First argument was that LM358 wasn’t the best choice for this purpose. Another one pointed to sine wave that weren’t smooth enough.
As you can see there are some dents on it. Other waveforms also are distorted especially when higher voltages are selected. This definitely asks for better analog part. Some people suggested to replace LM358 with OPA2134, but it seems to be quite expensive choice. In my opinion low noise general purpose op-amp can be great too. I’m gonna give a try to Texas Instruments TL074 low noise op-amp. It is low power, high slew rate (13V/us) IC – almost five times faster than LM358 and for same reasonable price.
Modeling of analog part for DDS3 signal generator - [Link]