UVM CricketSat Wireless Sensor

Remote Temperature Measurement

• Possible Application

– Measure temperature in the upper atmosphere

– 10 km altitude

• Two challenges:

– Measure temperature indirectly

– Long-range wireless transmission of data

Thermistor Temperature Sensor• Resistance changes with

temperature• Problem: Resistance

cannot be measured directly

• Requires additional circuitry to produce an electrical response

• Oscillator circuit is the simplest and lowest cost solution

• Produces an oscillation that changes with temperature

• Circuit based on the popular 555-Timer IC

• Frequency determined by two resistors and a capacitor

• Resistive and capacitive type sensors may be substituted

• In our case, the upper resistor is replaced with a thermistor

Temperature Sensitive Oscillator

555-Timer Oscillator Demo

Click on picture for online demo

Frequency vs Temperature

• Thermistor resistance increases with colder temperature, reducing charging current

• Charging time increases, reducing the frequency

The Wireless Connection

• Timer output signal enables transmitter during charging interval of the timing cycle

• Oscillator frequency is mixed with RF carrier frequency• Antenna is sized for carrier frequency -> short

Receiving Station

• Receiver extracts oscillator frequency

• Frequency measured by instrument or software

• Calibration charts used to determine temperature

CricketSat Sensor

•Oscillator frequency determined by temperature

•Oscillator output signal modulates RF carrier frequency

C1 C3C2C4 C5 C6

U2R1

D1DIP

Socket

Notch

U1: 555 Timer

IC

Dimple

Pin 1

Flat Side Up

1 2 31 2 3 4

8765

1234

U3: RF Transmitter

5-Volt Regulator

10K Ohm Thermistor

Printed Circuit Board (PCB)

47 micro-Farad Electrolytic Capacitors

Longer Lead

+ - + - + -

Longer Lead+-

Light Emitting

Diode (LED)

0.1 micro-Farad Capacitors

+-

Black Band D1 - Diode

+-

B1: 9-Volt Battery

Pins: Pins:

Antenna Wires

Battery Snap Connector

+

-

Red Lead is Positive

R4: 100 Ohm

R2: 3300 Ohm

R3: 680 Ohm

Brown-Black-Brown-Gold

Orange-Orange-Orange-Gold

Blue-Gray-Brown-Gold

Completed CricketSat

Prototype Development

Area

Thermistor

Test Points

Dipole Antenna

RF Transmitter

Module

Timing Capacitor

5-Volt Regulator

IC

555 Timer

IC

Flashing LED

9-Volt Battery Leads

Timing Resistor

Primary Features

Output Jack

Noise Decoupling Capacitor

Series Isolation Resistor

Current Limiting Resistor (LED)

Noise Decoupling Capacitor

Protection Diode (Reverse Battery

Connection)

Noise Decoupling Capacitors

Secondary Features