Guido knew the secret to a perfect cuppa
It’s all in the timing
So he designed a battery-powered tea timer with a built-in LED display that will brew a perfect cup of tea every time. It’s simple: just fix a tea-bag on the tea timer’s extension arm, place a mug of hot water underneath it and push a button according to how strong you like your tea. The microcontroller-operated, 5V circuit provides a servo motor with control pulses to lower and raise the arm and the power supply is switched off automatically via FET. Now the only thing you have to worry about is Tetley’s or Earl Grey?
Never forget your tea bag in the water again! Make your perfect cup of tea no matter what is around you. Just fix a tea bag on the extension arm, put a cup of hot water beneath it and push a button according to your desired time. Lay back or do what ever you want, the battery operated machine does the rest. It moves the tea bag down, lifts it up after the time is over and turns off automatically. The only thing for you to do is drink the tea.
What the circuit does
This microcontroller -operated circuit recognizes via push-buttons a desired time for a tea bag to be put into hot water, provides a servo-motor with control pulses to move an extension arm down and, after the time is over, up again. The power supply is switched off automatically via FET. The STOP Button interrupts the process at any time, a LED shows the tea timer is on duty.
How it works
First the user pushes one (or more) of the push buttons S6, S7, S8 for at least 3 seconds. The FET Q1 is shortened and the voltage-regulator IC2 provides 5V to the circuit (please note Label 1 and Label 2 on the circuit diagram, which means that every point marked with the same label-number has to be considered as connected, nothing more than a relief in drawing the schematic).
The microcontroller starts to work:
First Port RA1 is set to high for driving the FET Q1. Second, the controller reads the push button-ports RB0, RB1, RB2 in order to decide how long the teatime has to be performed. Note that not only one push button but any combination of them is possible. This enables the user to choose times from 2 minutes (only button S6 pushed) up to 9*minutes (button S6, S7, S8 are pushed simultaneously). Port RA0 is set to high and the LED indicates that the circuit is on duty.
The user releases the buttons but the circuit is still supplied with 5V because of Q1 who is still activated via RA1 and T1.
A couple of pulses on RA2 are generated to make the servo move down and a timer is started to wait for the desired time.
After the time has elapsed (and no STOP -Button is pushed in the mean-time) some pulses on RA2 make the servo move up again, RA1 is set to low and Q1 switches off the whole circuit.
*9 minutes are perfect if you caught a cold and need some herb tea, even though some people prefer a glass of whisky in this situation.
S1 is nothing but a slider to avoid -if opened- an activation of the Tea-Timer during transport or by little children. It is regarded as to be closed in the following considerations.
Q1 is a P-Channel Enhancement-Mosfet, simply used as a electronic switch to connect and cut off the battery voltage to the circuit. This means nothing but if the voltage between Gate and Source is close to zero or even positive the Drain-Source-Impedance is very high, nearly the same as if the connection between the Battery and the voltage regulator IC 2 is interrupted. That is when T1 shuts off, respective RA2 is low.
As RA1 (IC1) gets high (the program in the microcontroller tells him to do so right at the beginning of the working sequence) T1 is activated and the source-pin of Q1 is close to GND-potential. That means that the voltage between Gate and Source comes negative, the resistance between Source and Drain falls to nearly zero ohms. This is the most “tricky” part of the circuit.
R9 limits the current through the base of T1, C4 builds a low pass filter to avoid voltage spices and faulty switching-activities of T1.
IC2 is a voltage regulator and lowers the battery voltage from 9V to 5V, tuned with R11. A simple IC 7805 does the same work without tuning, I decided to use the LM317 just for some experiment
reasons and so it remained in the circuit…
IC1 is a PIC 16C54, the microcontroller does all the “intelligent” work in the Tea Timer.
R6 and C2 tune the internal Oscillator to approximately 580 kHz clock frequency. I kept it as low as possible to reduce power consumption and increase battery life. R7 and C3 make the Controller to start “save” and “reliable”. The microcontroller is only enabled if the voltage at Pin 4 has reached high level.
Ports RB0 to RB3 are connected to high level as long as no button is pushed. If it is, low level appears on the port pin. R1 to R4 limit the current while a button is pushed.
Port RA0 controls the LED, R5 limits the current. I preferred a yellow 3mm -LED because I just had one left in my spare box.
The Servo-Motor has 3 connections: +Vdd (5V), GND and a control-input.
Vdd is filtered by a 470 uF capacitor in order to avoid voltage-dips when the motor starts to run.
The control-input requires a PWM (Pulse Width Modulation) signal: Every 20 ms a pulse of various width controls the movement of the motor. The pulse width decides in which direction and how many degrees the motor turns his axis. Approximately a 1 ms-pulse makes the motor move into the top position (tea-bag out of water), 2 ms pulses make it to move to the down position (tea bag in water). This is about 90°. I decided to use the cheapest servo I could buy (about $5/Euros), it is strong enough to pull a wet tea bag out of the water.
Even though you needn’t to be an electronic expert, this is not a project for beginners. Some basic
tools and skills in mounting and programming a microcontroller as well as working with electronics are very useful.
Be careful with the polarity of the capacitors (if they have one) and make sure that you implement the fuse. Batteries may provide a lot of current which can do a lot of harm in a case of failure (may be a short circuit on the board).
The whole parts are mounted in an old cookie -box. I fixed the servo and the proto-board with some screws after having drilled the holes in the box. Note that the time-push-buttons are directly mounted on the board, facing the backside of the box. The battery-pack is wrapped in a piece of foam rubber and placed on the bottom of the box. The extension-arm is made of a piece of ledge. Be sure to put some cuts on it to fix the tea-bag in various positions.
Ideas for improvement
First remove that old PIC 16C54. There are many modern controllers (e.g. 68HC908 from Motorola, coming with an excellent C-compiler called “Code Warrior”) which can do the job with low power consumption, a comfortable way of programming and many features on board. Of course you have to adjust the circuit and the software according to the new controller.
Second substitute IC2 with 78M05, which needs no tuning potentiometer. Make sure to adjust the circuit in a proper way (see data sheet). The piece of software figured out further below should be
considered as a suggestion only. Feel free to program very smooth movement of the servo motor to achieve a gentle down and up movement of the tea bag or some other features:
What about some LEDs to indicate the progress of time?
What about a temperature sensor to check the water in the cup (provided that you like a sensor in your tea!)?
Think of an acoustic signal when the procedure is finished (if your environment allows)…
|Quantity||Part||Part description||RS Part no|
|2||D1, D2||BAV 100 Diode||2509049931|
|5||R1, R2, R3, R4, R7||10k Resistance||477-8246|
|2||R5, R9||4k7 Resistance||506-5210|
|4||S5, S6, S7, S8||NO Push Button||2509692712|
|1||Q1||Si 9430DY MOSFET||858-253|