I just realised Digikey has Attiny85 is back in stock (in december 2022). So now it's good time to build that ISP (In-System programmer) I've been thinking about for... some years. No more breadboard wiring mess or upload fails due to bad connections. Let's build a shield!
To be honest this is not really a shield. I think that would require a proper printed circuit board. But I'm not gonna bother with that, I build my shields to perf-board if I want to! As it happens, 4x6cm perf-board is perfect size for Arduino Uno.
This circuit is nothing special or new. The idea is to use Ardiuno UNO (or some other AT328 based system like Arduino Nano) as ISP (In-system programmer). 10uF capacitor is needed to prevent auto reset so that avrdude is talking to ArduinoISP sketch instead of Arduino bootloader. Resistors and LEDs are not needed and are optional. Red led is power indicator and green led will flash during upload/read.
While this short article is not about programming ATTiny-series microcontrollers, here are great instructions how to do that. Unlike these instructions I use ATTinyCore by Spence Konde. You can use either one. Just remember to burn bootloader (set the fuses) for required clock speed and type. If you dont do that, your ATTiny will run on internal 1MHz clock. And one more thing to remember: use "upload using programmer" from the Sketch-menu.
Anyway, this is simple and very useful little device. Quick to build too! I think you can expand this by adding more sockets for other ATTiny-series microcontrollers. I didn't do that since 85 is the only one I use.