I used Basom to program the Tiny13. The source is so short I can show it here. I used an integration time of 10 µs. At the beginning I collect a reference value Ref to know the LED voltage without signal. This is used later to detect the voltage loss caused by an RF signal.

Bas and HEX files: http://www.elektronik-labor.de/AVR/Sparrow/SparrowRX.html

Cheepit upload via Sound: http://tiny.systems/categorie/cheepit/RFDetector.html

Video:

The project goes on. Leander made a simple transmitter: The ATtiny13 generates an RF signal around 300 kHz and is tuned to match the LC circuit. A second LC circuit receives the RF energy. He also built a simple data link using two coils with hardly any additional components on the Tiny13.

'ATtiny13 RF receiver

$regfile = "attiny13.dat"

$crystal = 1200000

$hwstack = 8

$swstack = 4

$framesize = 4

Config Portb = &B000000010

Dim T As Byte

Dim N As Byte

Dim D As Word

Dim Ref As Word

Dim Diff As Word

Config Adc = Single , Prescaler = Auto

Config Timer0 = Pwm , Prescale = 8 , Compare B Pwm = Clear Up

D = 0

For N = 1 To 20

Portb.3 = 1

Portb.3 = 0

Waitus 10'integration time

D = D + Getadc(3)'LED2 voltage

Next N

Ref = D / 20

Do

Portb.3 = 1

Portb.3 = 0

Waitus 10'integration time

D = Getadc(3)'LED2 voltage

If D >= Ref Then

Pwm0b = 0

Else

Diff = Ref - D

If Diff > 2 Then

Pwm0b = 128'sound and LED1

Waitms 50

End If

End If

Loop

End