Infrared Transmitter:
 
 

A circuit diagram for an infrared (IR) transmitter was found on-line, designed by Mr. Ryan White. This design calls for an IR LED, two 555-timers and a collection of resistors and capacitors. This design modulates the IR transmitter at 45 kHz, to trigger the decoder located on the IR receiver circuit. This carrier signal will modulate within a square-wave
envelope of 600 Hz. This is to enable the GP1U52X (Radio Shack Receiver) since it scans for an incoming signal, modulating between 100 and 1000 Hz
 
 

Both timers are set up for a 50% duty cycle. The following analysis begins with the second 555-timer, configured as an a-stable oscillator. NOTE: when the capacitor is charging,
Vss = Vcc and the timing equation is used to find the charge time of the capacitor; which in-turn equals the length of one pulse.

T₁ = t_l ln [ (V_ss - V_I)/(V_ss - V_f) ]

T₁ = (Ra + Rb) C { ln [ (V_cc - V_I)/(V_cc - V_f) ] } Ra = 1kW Rb = 33kW C = 470pF

T₁ = (1kW + 33kW ) (470pF) { ln ( [ (V_cc - 1/3V_cc)/(V_cc - 2/3V_cc) ] }

T₁ = (1kW + 33kW ) (470pF) ln 2

T₁ = 1.1 x 10^-5 sec

For the discharging capacitor, assume Vss = Vce sat = 0.2 V to complete our analysis. The timing equation is then used to calculate the discharge time.

T₂ = t₂ ln [ (V_ss - V_I)/(V_ss - V_f) ]

T₂ = Rb*C { ln [ (V_cc - V_I)/(V_cc - V_f) ] }

T₂ = 33kW (470pF) {ln ( [ (0.2 - 10/3)/(V_cc - 5/3) ] }

T₂ = 1.2 x 10^-5 sec
 
 

Since T1 @ T2 the duty cycle is very close to 50%, where the error in T1 and T2 is within acceptable limits for proper operation. This 45 kHz. oscillating pulse will appear on pin 3 of the 555-timer and should look like the waveform below:
 
 

The first 555-timer pulse overlays the second 555-timer pulse - creating an envelope for the carrier signal. Again using the timing equation, the envelope frequency is calculated.

T₁ = t_l ln [ (V_ss - V_I)/(V_ss - V_f) ]

T₁ = (Ra + Rb) C {ln [ (V_cc - V_I)/(V_cc - V_f) ] } Ra = 1kW Rb = 33kW C = 33nF

T₁ = (1kW + 33kW ) (33nF) {ln ( [(V_cc - 1/3V_cc)/(V_cc - 2/3V_cc) ] }

T₁ = (1kW + 33kW ) (33nF) ln 2

T₁ = 7.8 x 10^-4 sec

T₂ = t₂ ln [ (V_ss - V_I)/(V_ss - V_f) ]

T₂ = Rb*C {ln [ (V_cc - V_I)/(V_cc - V_f) ] }

T₂ = 33kW (33nF) {ln ( [ (0.2 - 10/3)/(V_cc - 5/3) ] }

T₂ = 8.3 x 10^-4 sec

This creates the following waveform:
 
 

The final output to the IR LED will now resemble:
 
 

To make the transmitter portable, a 5-volt regulator is connected to a 9-volt battery. Lastly, a 24 W resistor is placed in series with the IR LED, to limit the current and control the voltage. Using this small resistor gives increased output and dramatically increases the range of transmitter. However, the batteries drain much faster.
 
 
 
 

Infrared Receiver:
 
 

Mr. Ryan White also designed this circuit. The GP1U52X IR detector obtained from RadioShack, first demodulates the incoming signal and then checks to see if the modulated frequency is within range. This is accomplished as the 555-timer recharges a capacitor through a resistor. If indeed the capacitor recharges, then the incoming signal is too slow, meaning that the IR received is not the signal emitted by our IR transmitter.