This is a simple assembly language program (approx 90 bytes) that plays the
classic music used in classic horror and sci-fi films.
The frequency is based on the amount of light hitting the LDR-CDS cell, which
you can vary by waving your hand over the LDR.

[code]
;-------------------------------------;
; A BASIC THEREMIN AETHERPHONE ;
; PLAYS NOTES ACCORDING TO THE LIGHT ;
; SENSITIVE ON-BOARD CDS CELL ;
; BY RETRODAN 7-OCT-09 ;
;-------------------------------------;

.include "m169def.inc"

.DEF A = R16
.DEF AH = R17
.DEF B = R18
.DEF C = R19
.DEF D = R20
.DEF I = R21
.DEF J = R22
.DEF K = R23
.DEF N = R24

.ORG $0000
RJMP RESET

RESET: LDI A,HIGH(RAMEND) ;SETUP THE STACK POINTER
OUT SPH,A ;AT TOP OF MEMORY AND
LDI A,LOW(RAMEND) ;GROW DOWNWARDS
OUT SPL,A
LDI A,128 ;SET CLOCK SPEED
STS CLKPR,A
LDI A,3 ;0=8MHz 1=4MHz 2=2MHz 3=1MHz
STS CLKPR,A ;BUTTERFLY RUNS AT 2MHZ OUT-OF-THE-BOX

LDI A,2 ;SELECT INPUT 0=TEMP,2=LIGHT
STS ADMUX,A

LDI A,0x83 ;(1<<ADEN) | (1<<ADPS1) | (1<<ADPS0) ;set ADC
prescaler to , 1MHz / 8 = 125kHz
STS ADCSRA,A

SBI PORTF,PORTF3 ;INTIALIZE PORT F
SBI DDRF,PORTF3 ;

READEM:
LDI A,0xC3 ;(1<<ADSC) | (1<<ADEN) | (1<<ADPS1) | (1<<ADPS0)
;set ADC prescaler to , 1MHz / 8 = 125kHz
STS ADCSRA,A........;START ANALOG TO DIGITAL CONVERSION
DISPLAY:
LDS A,ADCSRA ;WAIT FOR ADC CONVERSION
ANDI A,(1<<ADIF)
BREQ DISPLAY

LDS A, ADCL ;MUST READ ADCL BEFORE ADCH
LDS AH, ADCH
RCALL SOUNDER ;MAKE MUSIC
RJMP READEM

;================= SUBROUTINES ===================

;----------------------------------------------------;
; SOUNDS A FREQUENCY BASED ON CONTENTS OF A REGISTER ;
;----------------------------------------------------;
SOUNDER:SBI DDRB,5 ;SET PORTB-BIT5 FOR OUTPUT
BLUPE: SBI PORTB,5 ;SET PORTB-BIT5
RCALL BPAUSE ;WAIT
CBI PORTB,5 ;CLEAR BORTB-BIT5
RCALL BPAUSE ;WAIT
RET

BPAUSE: PUSH A ;PAUSE TWEEN PULSES
BPLUPE: INC A ;IE DETERMINS FREQ
BRNE BPLUPE
POP A
RET
[/code]