Replacing the Clock Resistor - Process and Example

Many children's toys that make electronic sounds contain a 'clock resistor'. This is a discrete resistor that is usually a surface mount component, and regulates the playback rate (pitch and speed) of the sound and all other time-based functions, including aspects like LED flashing rate and triggering interval of button inputs.

To locate the clock resistor:

• Remove the circuit from the housing of toy
• Continuously make sound with the toy. 
• During audio playback, run fingers over the components and listen to any variations in playback rate. 
• Narrow down this change to one component (or the connection between two point on the board). 

Once the clock resistor has been found, a number of different modifications can be made:

• A physical pot can be added to the circuit at the two points where a change of pitch was detected. The physical pot will only change the playback rate up (and not down). 
• The clock resistor can be removed by desoldering carefully. The clock resistor can then be replaced with a physical pot - the middle leg and one outside legs are connected to the two points where a change of pitch was detected.
• Instead of using a physical pot, a light dependent resistor can be used. 
• Instead of using a physical pot, a flex sensor can be used.  
• Instead of using a physical pot, a digital potentiometer can be used.
• Instead of using a physical pot, a series of pots wired as a sequencer can be used.

In the example shown below: 

• The circuit board of the toy is removed from the housing
• The clock resistor is located
• The clock resistor is removed via desoldering
• Two new connections are soldered on in place of the clock resistor. This is achieved by added some solder to each point on the circuit board, and the inserting a small wire per connection. 
• Note how much of a playback rate change can take place when the clock resistor has been removed 
• A light dependent resistor is added, and now controls the playback rate