GB2200232A - Battery powered temperature alarm - Google Patents

Abstract

The temperature alarm comprises a battery powered electronic device which is capable of monitoring temperature conditions for long periods of time, with a sounder which emits an alarm signal to advise of temperature changes outside preset limits and to advise of a low battery state. There is provided an audible warning controlled by an integrated circuit which causes an intermittent sounding to advise when the battery has fallen below a safe working voltage, Fig 4, not shown. A light emitting diode (Q1) indicates a normal working condition by flashing in a regular period of time. Thus power wastage by having to illuminate the light emmitting diode permanently is eliminated, Fig 2, not shown. Temperature is sensed by a thermistor circuit balanced to utilise practically zero amperage, Fig 3, not shown. A jack plug socket provides for connection to a separate sounder/alarm box where a remote alarm is required. This separate alarm box would have a separate battery which is inert until switched to the alarm state by the primary unit. The invention provides a means to use small batteries as a reliable power source to monitor a temperature for a long period of time with an audible warning of battery exhaustion. <IMAGE>

Description

BATTERY POWERED TEMPER4TURE ALARM This invention relates to a battery powered electronic device which is capable of monitoring temperature conditions for long periods of time, with a sounder which emmitts an alarm signal to advise of temperature changes ou outside preset limits and to advise of a low battery state.

Bacharound Temperature monitors using a thermistor and a balanced electronic circuit are not uncommon, utilising a primary power source from mains electricity frequently supported by a battery cell to supply power as a precaution against mains failure This format involves relatively expensive circuitry to ensure safe reliable use.

Alternatively a purely battery powered unit would normally require a large battery to allow for continuous current drain on the sensing circuit whilst still retaining sufficient energy reserve for operating the al arm circuit when required. A suitable battery to ensure supply- over several months would be expensive and large.

Essential Technical Features According to the present invention there is provided an aud ble warning coat@olled by an integrated circuit which causes an intermittant to to advise when the battery has fallen below a sate working voltage.

@ @@@ht @mmitting diode indicates a normal working condition by flashing in@@ -@qular period oi time. Thus power wastage by having to iliuminate the light emmitting diode permanantly is eliminated.

The thermistor sensing circuit is balanced to utilise practically zero amperage.

A jack plug socket provides for connection to a separate sounder/alarm bo where a remote alarm is required. This separate alarm bo would have a separate battery which is inert until switched ta the alarm state by the primary unit.

The invention provides a means to use small batteries as a reliable power source to monitor a temperature for a long period of time with an audible warning of battery exhaustion.

specific enbodiment of the invention will now be described by a- 0+ example with reference to-the accompanying drawings in which:. Fig 1 shows the general arrangement of the equipment with, ine temperature sensor connecting to the electronic controller incorporated in the alarm sounder box and a separate unit connected by jac plug for a remote alarm sounder facility.

Fig 2 illustrates the circuit which produces a pulsing flash trom a light emmitting diode using very low current in order to advise that the temperature alarm is on and that the temperature is being monitored.

Fig 3 illustrates the circuit used to monitor the temperature using very low current.

Fig 4 illustrates the circuit which monitors the voltage of the battery system. H switching signal irom the flasher circuit is used to provide a pulsating alarm on the sounder when the voltage falls below a preset value.

Fig 5 illustrates the circuit for the remote sonder Refering to fig 1. The sounder bo is affixed by self adhesive pads to a position close to where the environment requires its temperature monitoring. The temperature sensor is placed in the said environment and the unit is switched on when the normal operating temperature has been attained by the thermistor probe.

The light will commence flashing at reqular intervals to illustrate the equipment is functioning.

When the temperature of the environment being monitored falls below or rises above the limits set the circuit in fig 3 switches to provide power to the alarm sounder or to the remote sounder as required.

Since the equipment is designed to last for several months between battery changes, an alarm feature is incuded (fig 4) which sound:- an intermittent tone in synchronisation with the flashing light emmitting diode to warn that the battery requires changing.

CIRCUIT DESCRIPTION The circuit is divided into three sections:1) The Flasher Section. (fig.1) 7i The Temperature Monitoring Section. (fig.2) 3) The Low Battery Monitoring Section (fig.3) Description of the Flasher Section The terms high and low refer to the voltage level at the described point, high means the voltage is equal to the battery voltage whilst low ref eres to 0 volts.

On switch on the capacitors C1 and C2 are both discharged thus the output of iC1 is high and the output of IC2 is low. C begins to charge via R4 and the output of IC1, C1 also begins to charge but cannot rise above approximately 1.5 to 2 volts because of the cathode of D1 being held low by the output of IC2.

D1 will not be illuminated because the current through Di will be limited to a few microamps t R1. When the voltage on C rises to just above the voltage set by the potential divider R2 and R3 the output of IC goes high all-owina the voltage on Cl to rise above 1.5 to 2 volts as Di is now reversed biased.

When the voltage on Ci now rises above the voltage set by the potential divider R2/R3 (approximately 2/.--. of the battery voltage) the output 0-;' IC1 goes low thus C now begins to discharge via R4 and the output of IC1. When the voltage on C now falls below the voltage st by R'2/R7.-- then the output of IC goes low.

As C1 is is now charged up to above the voltage set by the potential divider R2/R3 the LED Di is now forward biased. The chars stored in C1 is used to give sufficient power to illuminate the LED Di as C1 is discharged. The voltage on C1 new drops to approximately 1.5 to 2 volts causing IC1 output to again go high as this voltage is below the potential divider voltage of R2/R3.

This oscillitary action then begins again where R1 and Cl determine the tiE between flashes of the LED and R4 and C2 determine the time the LED is on.

The value of R4 ad C2 is determined more by the time required by the low battery circuit to sound the low battery warninq as the LED discharges C1 in a fraction of the time that the low battery sounder is on. The output of ICI is used as a switching signal for the low battery circuit as the low battery circuit will only work when the output of IC1 is low this is explained in more detail in the low battery circuit description.

Descrintiij; of the Temperature Monitoring Section The resistance of P12 varies with temperature in such a way that as the temperature increases the resistance of R12 decreases thus the voltage at the junction of R5 and Ri2 decreases as the temperature increases.

The variation in voltage is compared with the potential divider R6 and RB and the potential divider R7 and R9, If the voltage at the junction of R5 and R12 drops below the setting of R7/RW then the output of IC4 goes low if the voltage at the junction of R5 and R12 goes above the setting of R6/R8 the output of IC3 goes low.

When the output of IC3 or IC4 goes low then the transistor T1 is biased on thus switching on the piezo electric buzzer and sounding the alarm.

IC3 or IC4 can be used on their own to detect either an over temperature or under temperature state and may be used in applications such as a freezer over temperature alarm, photographic developer over or under temperature alarm or any process where the temperature has critical limits.

If a remote sounder is required then the transistor T1 is used to switch on another transistor in the remote sounder thus both buzzers can be used to signal a temperature alarm or a low battery alarm.

Description of the Low Battery Monitoring Section D2 and D3 are used to provide a reference voltage which is constant irrespective cf the voltage applied to R13 thus as the battery voltage changes then the voltage accross D2 and D3 will remain fairly constant.

To minimize on current the diodes are biased with only a very small forward bias current but the voltage accross D2 and D3 is still sufficient to be used as a reference voltage.

R14 and R15 form a potential divider and as the voltage supplying Ri4 and R15 falls as the battery drains then the voltage at the junction of R14 and i5 will also fall.

When this voltage falls below the voltage accross DO and D3 then the output of IC5 will go low switching transistor T1 on and sounding the buzzer. The ratio of R14-and 1 determines the battery voltage at which the alarm will g off.

This voltage should be such that there is still sufficient power in the battery to sound the buzzer for several hours if a temperature alarm is required at the same time and yet should be low enough so that a long battery life is utilised.

To distinguish between a temperature alarm and a low battery alarm the low battery circuit receives a switching signal from the flasher circuit to cause the alarm to sound intermittantly. The signal from IC1 of the flasher circuit is connected to R15 which forms the lower half of the potential divider. The potential divider R14 and R15 can onlv biased to sense the battery voltage when the output of IC1 goes low thus the buzzer will only be sounded to indicate a low battery as IC1 goes low.

This results in a short 'peep' on the buzzer every few seconds signaling that the battery voltage has fallen below the preset value.

It should be noted that the the transistor T1 and the buzzer P1 are the same as in the temperature alarm circuit.

Claims (4)

1) h battery powered temperature alarm which is capable of monitoring 'temperature for long periods of time (months), with a sounder to advise of temperature changes outside oi preset limits and an alarm feature to advise of a low battery state.

2) h battery powered temperature alarm unit as in claim 1 not requiring outside power sources, nor battery charging features within its design.

3) A battery powered temperature alarm as claimed in claim 1 or claim which monitors the state of its own battery.

4) A battery powered temperature alarm as in claims 1,2 or 3 above which incorporates a low current using flasher to advise its monitoring state