A Knock Clock is a electro mechanical clock which tells the 'knocker' the time in audible 'knocks' to +/-10 minute accuracy. My better half saw one of these recently and asked me to make her one. This is my own implementation based on a circuit made by a member of the Manchester Hackspace - Paul Plowman!
The device itself is a basically a box with some electronics and a device for striking the side of the box to create the 'knock' sound. In this case I am using an electromagnetic plunger known as a solenoid. It is normally used to control valves or mechanical devices from an electronic controller. An electro-mechanical relay is essentially made up of a solenoid and a switch. More information about solenoids can be found via the internet and wikipedia!
Wikipedia's entry on Solenoids
The basic idea of the circuit is, as with most electronics:
- Take an input (the user 'knocking on the enclosure).
- Process this input (tell the processor to output the current time).
- Provide some form of output (cause the solenoid to actuate the time in hours and tens of minutes).
In this case the input is going to be sense the user knocking on the enclosure using a piezo buzzer as a simple microphone. Piezo crystals are a useful electrochemical crystalline structure which when force is exerted on the structure a high voltage low current signal is generated. It is the main component used is electric stove gas lighters and certain types of cigarette lighter. More wikipedia below:
Once the input has been detected it is necessary to process this input and then provide an output. To this end I am going to use the ever popular Atmel 328P microcontroller with the arduino bootloader. However I am not going to design a shield this time as I wanted to try to keep the cost of construction down. Using an arduino main board for every project can get expensive - even using clones is becoming expensive! So we have an input which we then need to compare to a known value - time! How does the microcontroller know what the time is? We could set the time on the microcontroller via the serial terminal each time the device is connected to a computer or we could use an ethernet controller and use the same technique; we could hard code the time from when the microcontroller was programmed and use that - best make sure the power is never removed! Or we could use a real time clock...I have decided to use a real time clock as this is in my opinion the best method to use ensuring that the clock keeps reasonable time.
Real time clocks are special integrated circuits which communicate via the I2C protocol directly to the microcontroller and are battery backed up which means that they are constantly powered by a small battery and never lose the time (once it has been set) even if power to the main circuit is removed. The accuracy over time is not always great (they lose a second each month) and the battery normally lasts about 5 years. There are several real time clock modules available to buy and rather than implement the circuit for myself I have bought one of these modules to save time. The real time clock device used in the module is based on the ubiquitous DS1307 by Maxim Semiconductor.
The module I am using was provided by the excellent and very helpful Adafruit Industries:
DS1307 RTC Kit
Now that we have a method of receiving and processing the input it is necessary to drive the output - a solenoid. Solenoids are fairly common devices and can be treated in much the same was as a relay - although a power hungry relay! It often takes a lot of current to drive a solenoid and the circuit will need to provide significant power (voltage and current) in order to make the solenoid actuate. The best way to achieve this is to use a Field Effect Transistor. I have discussed FETS in previous blog posts so I'm not going to go through this again - please read my previous posts.
We need an N-Channel metal oxide silicon field effect transistor ( N-MOSFET) with the following parameters:
Vds - 12V - I have decided to use 12V as the main voltage input
Vgs threshold - up to 5V - we need a logic level FET capable of being driven from the microcontroller
Ids - 1A or higher - we need a device capable of driving the Solenoid
T0220 Package - I like big through hole parts - they are easy to solder!
There are probably over a hundred devices that meet these requirements. If we do a search using Farnell Electronic's parametric search facility the following results are found:
We could use any of these N Channel-MOSFET devices and the VGS threshold voltage is the most important parameter as we need the device to work from a 5V digital signal from the microcontroller. As we don't have any particular preference I am going to choose the least expensive device to keep costs down and the cheapest device is one I already own!
The device I have chosen is the IRF630 by ST Microelectronics - the datasheet is below:
It meets all of the requirements and costs a very reasonable £0.56 from Farnell Electronics.
So....we now have all of the parameters for the Knock Clock decided we can go ahead and design the circuit. I have decided to use a basic implementation of the arduino using an Atmel 328p. The circuit will not have the serial to USB converter or a method of programming the microcontroller so we will have to provide a method of programming and talking to the microcontroller. The rest of the circuit is standard boiler plate electronics - A 12V to 5V linear regulator, a piezo electric speaker (being used as a microphone) and a MOSFET driving a solenoid. The centre header is for the real time clock module. The circuit is shown below:
I have included a six pin programming header to initially 'flash' the arduino bootloader onto the microcontroller and a header to allow communication with the microcontroller via a USB to serial cable. The 12Vdc input will be provided via a standard dc barrel jack or via 5mm screw terminals. The output to the solenoid is provided also via 5mm screw terminals.
Here is the PCB layout for the bottom layer:
Here is the top layer with the component identification:
The bill of materials for this project is as follows:
|C3||0.1uF||Electrolytic Capacitor||Capacitor Polarized|
|C4||10uF||Electrolytic Capacitor||Capacitor Polarized|
|C5||10uF||Electrolytic Capacitor||Capacitor Polarized|
|IC1||LM7805||T0220 package||Voltage Regulator|
|J1||Programming Header||6x 0.1' pitch header pins||6 pin I2C programming header|
|J2||12Vdc power jack||3.5mm dc power jack||Power Jack|
|JP1||6 pin Header||6x 0.1' pitch header pins||Serial Communications Header|
|JP2||5 pin Header||5x 0.1' pitch header pins||RTC Header|
|JP3||5mm Screw Terminal||5mm screw terminal phoenix connector||12Vdc input from screw terminals|
|JP4||5mm Screw terminal||5mm screw terminal phoenix connector||Solenoid output from MOSFET|
|KK1||Heatsink||T0220 Heatsink||Heatsink for MOSFET|
|LED1||5mm Red LED||5mm Red LED||Power LED|
|Q1||IRF630||MOSFET N-CHANNEL||Common logic level MOSFET|
|R1||10k||1/4W axial resistor||Resistor|
|R2||220R||1/4W axial resistor||Resistor|
|R3||1k||1/4W axial resistor||Resistor|
|R4||100k||1/4W axial resistor||Resistor|
|R5||1M||1/4W axial resistor||Resistor|
|S1||Microswitch||momentary tactile switch||Momentary Switch for reset|
|SP1||Piezo Buzzer||Piezo Buzzer||Piezo Buzzer used as a microphone|
|U1||ATMEGA328P||ATMEGA328P - DIP package||Microcontroller with arduino bootloader|
|Y1||16MHz||16MHz crystal (through hole)||16MHz Crystal for microcontroller|
Just for fun I used an online gerber viewer to show what the printed circuit boards will look like if manufactured:
Here is the Bottom Layer
Here is the top layer:
Well that is about all for now. In the next post I will show pictures of the actual construction and then finally I'll go through the programming of the firmware for the microcontroller. Enjoy - Langster!