Electronic service timer for an appliance

A circuit is described for timing the intermittent use of an appliance, wherein the appliance includes a usage signal for indicating that the appliance is in use. The circuit includes a battery, and a resistive network, coupled to the battery, for draining power from the battery such that the power level thereof decreases to a threshold level over a selected timing interval. The circuit further includes a switch, responsive to the usage signal, for selectively enabling the resistive network, and a battery level threshold detector, responsive to the battery reaching the threshold level, for indicating that the timing interval has lapsed, thereby alerting the operator that the appliance should be serviced, which service includes replacement of the low cost battery.

FIELD OF THE INVENTION 
The present invention relates generally to electronic timing, and, more 
particularly, to the practice of electronically timing the intermittent 
use of a device, or appliance. 
DESCRIPTION OF THE PRIOR ART 
Known techniques for electronically timing the intermittent use of a device 
(appliance) may be categorized into three areas. The first area 
encompasses electric clock circuits. These circuits employ an electric 
clock which is enabled when the appliance is turned on in order to time 
the use of the appliance. The use time of the device is provided by the 
clock readout. 
The second area includes techniques employing metallic build up on an 
electrolytic plating cell. Metal is constantly deposited at the cathode of 
the plating cell when the device being timed is in use. The amount of 
plated metal is visible through the glass walls of the cell where the use 
time of the device can be determined from graduations. 
Third, electronic counters are used in a manner similar to the clock 
circuits, described above, to time the use of the appliance. An 
oscillator, operating at a fixed frequency, is provided to a counter while 
the appliance is operating, and removed while the appliance is idle. At 
the output of the counter, a "count" is used to determine the time the 
appliance has been used. 
Unfortunately, there are disadvantages associated with each of these known 
techniques. First, each of these techniques is relatively costly. This is 
especially true when attempting to provide such a timing function for a 
home appliance where cost is an essential factor. 
A second disadvantage is associated with appliances which use electronic 
timing methods and which require external power for operation. Such 
appliances require an internal battery to sustain the timing information 
while the external power to the appliance is disabled. If the life of the 
battery naturally terminates before the timing information is accessed, 
the timing information is forever lost. 
Accordingly, there is a need for a circuit for timing the intermittent use 
of an appliance which overcomes such deficiencies. 
OBJECTS OF THE INVENTION 
It is a general object of the present invention to provide an appliance 
usage timer which may be employed for cost conscious applications. 
It is a more specific object of the present invention to provide an 
appliance usage timer which alerts the appliance operator when the 
appliance requires service and which is not susceptible to failure due to 
natural battery life termination. 
It is yet an additional object of the present invention to provide an 
appliance usage timer which may be used over relatively long periods of 
time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention disclosed in this specification has particular use for timing 
of the intermittent use of devices (electrical, mechanical and 
electro-mechanical). More particularly, this invention has applicability 
for timing the intermittent use of an appliance and for alerting the 
appliance operator that a predetermined period of appliance use time has 
been reached, indicating, for example, that the appliance should be 
serviced. The invention is best suitable for such devices (appliances) as 
vacuum cleaners, lawn mowers, industrial machinery and boat motors, all of 
which are intermittently operated and require periodic service based on 
the operation time, but may also be used to time the continuous use of 
appliances. 
FIG. 1 illustrates a circuit 10 which provides for such timing. External DC 
power, not shown, is provided to the circuit at terminals 20, while a 
power cell 24, such as a Lithium or Alkaline cell (e.g. Manganese Dioxide, 
Mercuric Oxide, or Silver Oxide) is provided at terminals 22. A Lithium or 
Alkaline cell is preferred due to the low discharge rate associated 
therewith. In a modernly designed Lithium or Alkaline primary cell, the 
inherent low internal impedance and precisely controlled service capacity 
permit external resistance to essentially determine the current flow 
therefrom; hence, the discharge time. As will be described below, control 
of the discharge time of the power cell 24 is a central aspect of the 
present invention. 
When power is provided at the terminals 20, the appliance is considered to 
be in operation and the circuit 10 begins timing the appliance use. Thus, 
the presence of power at terminals 20 acts to signal the circuit 10 that 
the applicance is in use. 
The primary function of the power cell 24 is to act as a timer such that 
the power cell 24 is forced to discharge to a threshold level over a 
selected "use" interval. Another portion of the circuit 10 establishes the 
rate of discharge of the power cell and selectively controls when such 
discharge will occur. Preferably, the power cell 24 discharges only when 
the appliance is operating. When the power cell 24 discharges to a 
threshold power level, such as a voltage level indicative of the end life 
of the power cell, a transistor 36 activates an LED 26 to notify the 
operator that service is required. The power cell is replaced during such 
service. 
The discharge time is predetermined, measurable through conventional 
battery life tests, according to the power cell type. It should be 
apparent that the longer the shelf life of the power cell, with respect to 
its operating life, the greater the accuracy of the estimated discharge 
time. 
More particularly, in order to discharge the power cell 24, a primary 
discharge path is provided through a pair of resistors 30 and 31 and the 
collector path of a transistor 32. The base-emitter junction of the 
transistor 32 is forward biased by the external power at terminals 20 so 
that when the external power is disabled (when the appliance is no longer 
operating), the discharge path through the collector of transistor 32 is 
"opened" (effectively disconnected), thereby allowing the power cell 24 to 
discharge only during operational appliance use. 
The step of disconnecting the discharge path, through the collector of 
transistor 32 is accomplished by driving the base of transistor 32 by the 
external DC power. When the DC power is no longer present, transistor 32 
turns off. 
The power cell 24 is also used to forward bias the base-emitter junction of 
a transistor 34 while the primary current path (through the collector path 
of transistor 32) is "closed" whereby current is flowing through the 
current path. While this primary current path is closed, a current path is 
provided from the external DC supply at terminals 20, through a resistor 
39 and a diode 38. The base of transistor 36 is effectively grounded 
through the respective collector current paths of transistors 34 and 32 to 
prevent the LED from turning on while the power cell is forward biasing 
the transistor 34. 
After the power cell 24 discharges for a preselected amount of appliance 
operation time, the power level, i.e., the voltage and/or current level, 
of the power cell 24 decreases until it reaches a threshold level, at 
which point the base-emitter junction of the transistor 34 is no longer 
forward biased. When this occurs, the current path through the collector 
of transistor 34 and diode 38 (provided by the external power at terminals 
20) is "opened" and current through resistor 39 can then flow through the 
base emitter junction of transistor 36 so as to provide current from the 
DC power at terminals 20 through the collector path of transistor 36 and 
the LED 26, coupled thereto, to alert the operator that the appliance has 
been operated for the selected period of time, i.e. service is needed. 
When transistor 36 is turned on, emitter current flows through a resistor 
47 and a diode 48 to forward bias the base-emitter junction of a 
transistor 42 and to set a latch 49, comprising transistors 40 and 42 and 
resistors 43, 44, 45 and 46. Once the latch 49 is set it remains set until 
the remaining energy of the power cell 24 is exhausted, i.e., until 
insufficient current is available from the power cell 24 to hold the latch 
49 in the set mode. This latching is advantageous in that it prevents the 
power cell 24 from recovering to an operative power level. 
It should be noted that diode 38 is provided between the collector of 
transistor 34 and the external power to prevent the power cell 24 from 
discharging through the base-collector junction of transistor 34 while the 
appliance is not in use. 
In FIG. 2, a similar circuit is illustrated for application to an external 
AC supply at terminals 50. The left half of the dotted lines in each of 
FIGS. 1 and 2 illustrate identical circuits. On the right half of dotted 
lines, rather than directly controlling the base-emitter junction of 
transistor 32 with the DC supply (as in FIG. 1), the AC supply is 
rectified, through a diode 52 and a series resistor-capacitor 54-56 
arrangement, before coupling the supply to the base-emitter junction of 
transistor 32. 
Further, a neon bulb 58 is coupled from the junction of diode 38 and 
resistor 70 to the base of transistor 42 such that when the power cell 24 
drops below the threshold power level, transistor 34 is biased off, and, 
consequently, a current path is provided from the AC power at terminals 50 
through the neon bulb 58 to set the latch 49 and to alert the operator 
that the appliance is due for service. 
Accordingly, the present invention provides a low cost circuit arrangement 
for timing the intermittent use of an appliance and for alerting the 
operator that a predetermined usage time has been reached. Moreover, 
although the circuit requires a power cell, unlike those previously known 
inventions requiring a power cell, the present invention cannot fail as a 
result of the power cell naturally reaching its threshold voltage. 
It will be understood by those skilled in the art that various other 
modifications and changes may be made to the present invention without 
departing from the spirit and scope thereof.