Room Thermometer

Just add an A/D converter and a few discrete LEDs to this temperature sensor chip and you get a simple thermometer. This application builds an accurate and stable thermometer displaying room temperature with a resolution of 1 °C.The MAX6610 from MAXIM is a relative newcomer to the field of temperature sensing IC’s. The chip has a built-in voltage reference and an analogue output scaled to simplify integration with an 8 or 10-bit A/D converter (see textbox). A simple room thermometer does not usually require such precision and we run a risk of being accused of using a sledgehammer to crack a nut but the chip is quite versatile and this application provides a useful introduction to some of the features of the device. For more information on this device see the inset.

Standard three-legged voltage regulator chips like the LM317 can cope with an input voltage of up to about 30 V, a few high-voltage types can handle 60 V but if that is still not sufficient for your application the company Supertex (www.supertex.com) produce devices that can withstand much higher input voltages. The regulator type LR8 has a maximum input voltage of 450 V and can supply an output current of 20 mA. The LR 12 has a better output current of 50 mA but with a maximum input voltage of 100 V, and the output voltage can be adjusted up to 88 V.

Modern digital systems work with a supply voltage of +3.3 V, and sometimes they also need an additional, lower supply voltage, such as 1.8 V, 1.5 V or even 1.2 V. To generate a reset signal from these two voltages, it was previously necessary to use a separate reset IC for each voltage, and each IC had to be individually dimensioned for the voltage it monitored.

This circuit may be used to replace the single current limiting resistor often found in dirt cheap battery chargers. The alternative shown here will eventually pay off because you no longer have to throw away your NiCds after three months or so of maltreatment in the original charger.The circuit diagram shows an LM317 in constant-current configuration but without the usual fixed or variable resistor at the ADJ pin to determine the amount of output current. Also, there is no switch with an array of different resistors to select the charge currents for three cell or battery types we wish to charge: AAA, AA and PP3 (6F22).

The National Semiconductor LMV225 is a linear RF power meter IC in an SMD package. It can be used over the frequency range of 450 MHz to 2000 MHz and requires only four external components. The input coupling capacitor isolates the DC voltage of the IC from the input signal. The 10-kW resistor enables or disables the IC according to the DC voltage present at the input pin. If it is higher than 1.8 V, the detector is enabled and draws a current of around 5–8 mA. If the voltage on pin A1 is less than 0.8 V, the IC enters the shutdown mode and draws a current of only a few microampères.

It is often necessary in complex designs to provide a sequence of reset pulses to different parts of a circuit to ensure the whole design functions reliably. The DS1830 from Maxim (www.maxim-ic.com) provides three sequenced open-drain reset outputs. This chip is designed for 5 V systems but a 3.3 V version (DS1830A) is also available. Both are offered in a range of package outlines including DIP, SO and µSOP.

A rectangular-wave pulse generator with an extremely long period can be built using only two components: a National Semiconductor LM3710 supervisor IC and a 100-nF capacitor to eliminate noise spikes.This circuit utilises the watchdog and reset timers in the LM3710. The watchdog timer is reset when an edge appears on the WDI input (pin 4). If WDI is continuously held at ground level, there are not any edges and the watchdog times out. After an interval TB, it triggers a reset pulse with a duration TA and is reloaded with its initial value. The cycle then starts all over again. As a result, pulses with a period of TA + TB are present at the RESET output (pin 10).

Processor based systems usually require a voltage supervisor chip to produce a clean reset pulse to the processor whenever a ‘brown-out’ condition of the power supply is detected. More complex designs employing multiple power supplies can be unreliable if some of the supplies are not supervised. The circuit described here monitors all the supply rails in the system (here +12 V, –12 V and +5 V) and provides a reset pulse to the processor whenever it detects any are not within tolerance.

The gain of an operational amplifier is usually set using two external resistors. If you wish to have adjustable gain, you can use a digitally controlled multiplexer to select several different gain-setting resistors.Such an arrangement using several ICs can now be replaced by the Linear Technology LTC 6910 single amplifier or LTC 6911 dual amplifier. These ICs incorporate all of the gain-setting components and can be programmed to eight different gain settings using three digital control inputs. The amplifier is always configured in the inverting mode and features rail-to-rail input and output.

When it is necessary to send a digital signal between two electrically isolated circuits you would normally choose an optoisolator or some form of transformer coupling. Neither of these solutions is ideal; optocouplers run out of steam beyond about 10 MHz and transformers do not have a good low frequency (in the region of Hertz) response. The company NVE Corporation (www.nve.com) produces a range of coupler devices using an innovative ‘IsoLoop’ technology allowing data rates up to 110 Mbaud.